You might want to check the regulations on this sort of thing, that are specific to where you live, if it is your intent to comply with the regulations. Some areas have restrictions on power, frequency, modulation methods and distance, among other things. Once you know the envelope you are working in you can go from there.
Someone here had suggested Linx as a possible source of transmitters and receivers. Their site is quite interesting and it appears that much of what they have to offer is very reasonable in cost.