Where's the best place to buy parts, kits, ect? How hard Is it to do, what all is needed and what's the cost to build a good mid level Ar. I already have a rock river arms Ar 15 but I really want to look into building one. Any help pointers, brands to avoid, places to go, ect I'm located close enough to the fort worth/ Dallas area to get things needed.

Oh an to add I'd like a lower that can be used on multiple calibers not just the 223/5.56 NATO

Parts are out there and you can build an AR fairly cheap.
If you plan to build/tinker with them regularly, you will need about a 100.00 for tools and this will sometimes make or break a first build.
At a minimum, you would need a upper receiver block and an armorers tool, some punches and regular hand tools.

The uppers/lowers are universal if it's a true AR platform and interchange effortlessly.
True ARs are all cut from the same set of base dimensions and should swap out without a problem.
The only one I know that is different is the Colt which uses a slightly different pin size that joins the upper/lower together.
If is says Colt on it, it ain't gonna be cheap!

One of the shops from San Antonio advertised complete Plum Crazy lowers for 125.00 a few weeks ago in the Classified section..

I'm almost finished on my second AR build, a 300 AAC Blackout and only waiting for the gas tube to come in - it's been a miserable week waiting on it, too!