Not a stupid question at all. I assume you are referring to handloading? For factory ammo you just buy anything labelled 303 British.
Ideally, you would use Speer, Hornady or Sierra 311 or 312 bullets.
In today's market, bullets are hard to find, so you may need to use 310 bullets intended for the Russian guns, or even 308, if youc an't find anything else.
good luck.
Try a WTB on EE.