There are sources out there. Unfortunately, for us non reloaders (hopefully not me for much longer) .303 Brit runs about $1 per round!!
Here is one site that should help.
http://www.ammoengine.com/find/ammo/.303_British
The first choice on that list, Sellier and Bellot 180 gr FMJ from cheaper than dirt is a good deal. I have shot a bunch of that exact round, and its the best performing FMJ Enfield ammo that I have found.
Enjoy!!