I bought a .455 webley die set (Lee) from Wholesale Sports a few months ago for $40. It's a good set, i like it. Ellwood Epps probably has them too.
Mystic Precision can get .455 Hornady Mark II brass for you, which is made for smokeless powder. Send a PM to mysticprecision, or click on his banner at the top of the page. I've reloaded for half a dozen webleys using Unique powder, which theoretically produces less pressure than black powder.
smellie is correct that a longer case is better for black powder. .45 auto rim or .45 long colt brass can be made to work. .45 auto rim is about impossible to get right now. .45 long colt is easier to find. Both need the rim thinned to fit in an uncut webley .455, and the colt will need the case length shortened as well.
OMA products in BC is the only Canadian maker of proper Round Nose Hollow Base .455" webley bullets that I know of. They are pricey but the only other choice is to cast your own. Some people have reported good results using .454" bullets made for the .45 colt. That would be good because you can get .454" cast bullets for less than half the price of RNHB webley bullets.