Not have a good idea of exactly what kind of signal source you require, here's an article devoted to a basic overview of different types of generators. From that, maybe you can zero in on a specific type.
Generators
"Generator", specifically "signal generator", is a very loose term in general that can apply to any item of equipment that outputs some predetermined, usually alternating current, "signal" or waveform. Except for very specialized generators, the waveform is usually repetitive\ and, variable in frequency and output voltage. Here are some typical types of generators.
Signal Generator. This is a generator designed for communications work, such as aligning radio receivers. Usually called an RF signal generator, better models (laboratory-grade) produce pure sine waves with very little distortion and few harmonics from around 10 to 50KHz to maybe 500MHz. "Service-grade" RF generators have distorted outputs rich in harmonics. Models usually can be amplitude modulated and have attenuators and variable output level controls. Some upper-end models can be frequency-modulated and others pulse-modulated. Older models were usually analog generators and new models are typically digitally-synthesized generators. Good ones aren't cheap and prohibitively-expensive when new. I suggest obtaining a used lab-grade analog instrument for the best cost vs. utility tradeoff. The Hewlett-Packard 606A and 606B are considered to be the finest examples of analog HF (high-frequency) generators with a range of 50KHz to 50MHz, built-in 120dB attenuator, built-in or external AM or pulse modulation as well as CW (continuous wave) output. The 606 has a high-purity output with very little distortion. Expect them to be available from on-line auctions for anywhere from $50 to $200 depending upon the source and condition. You can often find cherries for $50.
RF generators are usually manufactured for various ranges of frequencies: HF, VHF, UHF, SHF, EHF, etc. The most common are HF and VHF.
Function Generator. A function generator outputs several different waveforms. You can usually select from at least a sine, triangle and square waveform. In addition, some also have ramp and "pulse" waveforms available. Most are able to DC offset the waveforms so that they can be made compatible with various circuits under test, such as TTL or CMOS digital circuitry. The frequency range of function generators range from about 1 Hz to 200KHz for the cheapest to 0.001Hz to 40MHz for the most expensive. Some have built-in sweep capability and voltage-controlled frequency (VCF) input. On model even has amplitude modulation capability. These are all-around good generators for general experimentation. The sine wave output of most is not the best – OK for testing purposes but impossible to use if you're doing any distortion checks. The sine wave of a function generator usually gets even more distorted in the upper frequency ranges. Some function generators have built-in low-end frequency counters.
Pulse Generator. Pulse generators can usually vary the frequency, duty cycle, low-level voltage, high-level voltage, rise time and fall time of a pulse waveform. Rise and fall times are usually very fast, in the nanosecond and even picosecond range.
Sweep Generator. Sweep generators are specialized versions of RF signal generators and have similar specifications to them. They can automatically vary their output frequency about a center frequency or from a start frequency to a stop frequency. They are used for alignment of communications gear, especially receivers, allowing a graphical representation on an oscilloscope of the characteristics of frequency dependent amplifiers (e.g., IF amplifiers) or of filters. In addition to the normal signal generator outputs, they also have a ramp or horizontal output that is used to drive the horizontal input of an oscilloscope for the swept display. Higher-frequency units often used detector probes to display the output as a DC waveform since most oscilloscopes in the days of those units had low bandwidth. Today's oscilloscopes, some capable of gigahertz bandwidths, can operate directly without the use of the detector.
Stereo Generator. For aligning FM stereo radios, a stereo generator is needed. This generator outputs the left and right channel signals for aligning an FM receiver.
Teletype Generator. This older unit generated the Baudot signals needed to adjust Teletype systems. They could vary the output data rate for different systems. In addition to an RYRYRYRYRY output, they usually had a "fox" test message that, in Baudot, outputed the mesage "The quick brown fox jumps over the lazy dog. 0123456789"
Arbitrary Function Generator. This generator usually has the basic waveforms of a function generator. In addition, it can output a custom waveform designed for exactly what you need in your testing. Once you're at this level of generator, you'll probably be using lots of automated test and measurement equipment.
Television Test Generator. This generator outputs a test signal for testing out television systems. Be aware that you must have the generator designed for the television system in use: NTSC, PAL or SECAM. This generator will quickly find its days numbered as digital television takes over. There will be specialized HDTV generators take over where these analog generators left off. The TV test generator will output test patterns such as red, green, blue or white screens, various types of color bars, gray-scale gradient patterns, etc. Most output a composite color waveform while some will also have these waveforms available as a standard television RF channel.
Dean