I have found in base TTW in the capital wasteland .308 is "somewhat" rare, sure you have hunting rifles and sniper rifle enemies but those guns rather outnumber the 5-8 rounds of ammo the particular foe will be carrying with it. Many vendors in the capital wasteland don't offer "bulk" amounts of .308 either, usually between 6-10 rounds, it should be noted that .308 seems to be a later game ammo for the FO3 portion of TTW. My best suggestion is for those wanting to use a Hunting Rifle/Sniper Rifle is make good use of a ammo bench and the hand loader perk, and convert all normal .308 you get into .308 JSP hand load, and break down other ammo you might not use as much, to me all .22 rounds get crunched automatically. Gribbleshnibit8 had made a really good mod I used with my TTW games, called "Vending Machines of the Wastes" http://www.nexusmods.com/newvegas/mods/60402/?
It adds a vending machine to several places in both the Mojave and the Capital wasteland that would allow you to make ammo purchaces in bulk, however at the added cost of greatly increased caps per item. With hand loader and reusing the .308 brass that will be recycled back into additional JSP rounds, I found that my overall ammo needs where met.