5 round magazine for p308?

You can modify a 20LR PMAG (the previous generations that is...not the new, M3 model which won't work because of the baseplate redesign) by cutting it down and pinning the baseplate back on. Search the forum as there is a "How To" here somewhere (if not here...there is one on BARFCOM that works in the same manner with the same reliable results).

If not that, then wait for the 10-round factory offering from Magpul (should be in the next month or so) and the 5-round limiters that they'll be making for them.

OR...if not either of those options...there are the DPMS limited capacity mags (4-round) seen here: DPMS Mag LR-308 308 Winchester 4-Round Steel Parkerized

Good luck!