Any chance of locating a nearby reloader than could provide some decent reloads?
Maybe visit a local target range/gun club... or post on the internet? Or, learn to reload your own .303s in the long term? That may prove practical in the long term.
I can't remember seeing surplus 303 since the late 90's
Any you do find will probably be really old, and corrosive.
I did a little searching online and the best I could find was sportsmans guide had fmj for $16 for 20. $150 for 200, plus shipping. That sellior and bellot of recent manufacture. Prvi Partisan may load 303 but I'm not sure. Big sporting goods stores may carry it.
Your best bet in the long run would probably start reloading for it.
Call Ammo Depot in Caddo Mills, TX or Military Gun Supply in Fort Worth, TX both have it all the time. As has been said it is very corrosive so clean your weapon with hot soapy water after shooting the stuff.
I found some at a gun show, they appeared to be surplus, but soft point bullets. Got 70 for 10 bucks, in a nice antique cigar box. No luck since. If you're gonna buy online, try http://www.ammoseek.com/ or http://www.ammoengine.com/ . This will save you a lot of shopping around.