I just purchased a 788 without a magazine. I see some aftermarket and a few Remington made mags for sale. Any difference in quality? My rifle is .308 Win. Can I use mags for the .243
and 7-08?

IIRC any 308-based cartridge mag for a 788 will work; I have two Remington mags, I can't vouch one way or the other on the aftermarket versions.

after market mags are a crap shoot at best. find an original and pay a few more $$...but it will work.