Tap, you really don't have to find a complete gun at once. Any MilSpec lower is going to mate with a MilSpec upper. Focus on finding a good, flat-top upper. Keep in mind that bull barrels get heavy quick but the mid-weight's have plenty of accuracy for what we're doing, and way more functional as a walking varminter. The lowers can be had from a slew of places, but at some point you'll probably want a better trigger.
If you're hesitant about doing any building yourself, there's a ton of information here and vids all over the net that cover all steps.... and you'll enjoy making it evolve on your own as you learn more about what you want.
Enjoy...