If you read the LM317 datasheet, it says that the way it works (as a voltage regulator or current regulator) is that it modifies it's output to maintain try and maintain a 1.25V between the output and adjustment pin. If you wire it as a current source, what it tries to do is push enough current through that resistor to always keep 1.25V across it. Constant voltage across a resistor means constant currrent, and since "zero" current flows into the adjustment pin, all the current in the resistor goes to the output making your current source.
It does this same thing when working as a voltage source, the only difference is that you have two resistors instead of one (look at the regular voltage regulator circuit in the LM317 datasheet). The LM317 drives the current through the resistor between Vout-ADJ to keep the voltage across that resistor at 1.25V, but since the current must flow through both resistors (no current flows into the ADJ pin), the result is that the voltage across both resistors is higher than 1.25V- your regulated voltage which is determined by the values of resistors used.
The other circuit works the same way.
If you understand how this works, then the operation of all linear current sources is very similar (hint: the way all op-amp circuits work is they modify their output so that the voltage and + and - input terminals are equal, including this case):
https://en.wikipedia.org/wiki/Image:Op-amp_current_source_with_pass_transistor.png
Remember that a current source has to be able to output a voltage that is AT LEAST as high as
V = Iconstant/Rload
So if you want 500mA going through a 2K resistor, this means that the current source has to output 2000V, and thus needs a power supply even higher than 2000V and components to withstand this voltage. Obviously, this is a very hard current source to find.
THe reason those current sources can only drive loads 1K-2K range is because their output voltage isn't high to be able to drive their maximum possible current through resistors any larger than that. So if you want a stronger current source (one that can drive more current through a higher resistance), then you need to have a higher voltage supply and components that can tolerate that- and that's the only difference, higher source voltages and components that can withstand that source voltage. That's how you select the components.
No buffer should be needed because the current sources ARE amplifiers/buffers wired with feedback to produce that output. If you used a buffer it would be like wiring two current sources in series- pointless. It work with voltage because voltage doesn't "flow", so buffers cascade to increase the voltage or buffer it or whatever. But current does flow, so if you cascade it, then the later stages just block the current from the first stage stopping it from appearing on the output. You'd have to use parallel current sources so all the current combines at the output, but then you have balancing issues between all the current sources in parallel since they are never identical and might start to fight each other. It'd be best to only use one stage and but use a amplifier that can handle more current for that stage.