Building is always better in my opinion... But, parts are pretty well nonexistent right now.
Your best bet if you have to have one now is a gun shop, they are less likely to gouge as much as a private seller. I really don't see you getting one at a reasonable price right now. Me personally, I would either hold out, or pick up an SKS, they are running about $400-$450 right now. Expect to pay $1200-$2000 for an AR plus AR mags are running $40+ each