Fiocchi ammo is reasonably priced and normally can be found online. I start by shooting purchased ammunition and reload the brass. Too much work trimming and thinning the base on other caliber brass, so if the gun is original, buy a few boxes of Fiocchi and you will have all the brass you need.
The bullet design is quite different from standard 45 Colt style bullets. I have three 455s and find that solid base bullets do not achieve the accuracy I would like. I purchased a RCBS bullet mold and it not only produces bullets with the skirted hollow base, it has dimensions of the original military bullet. For me, it is substantially superior to solid base bullets and the mold is worth the investment.
The key in powder selection is finding a powder that will get you under 700 fps and keep standard deviation low. Military rounds were in the 625 fps range and Fiocchi runs 262 fps in my revolvers. My choice of powder is Trail Boss. I load 3.5 to 4 g TB and do some test loads through a chronograph to get to the velocity of around 650 fps. No chance of double charge with TB and this powder offers very low SD, but there are many other good powders to chose from as detailed above.
Here is an article that might be of interest to you.
GUNS Magazine The .455 Webley: Part 1 - GUNS Magazine