Right now either Sinclair (Brownell’s affiliate) or Buffalo Arms has Hornady .500 3” brass for about $70 - $74 for a box of 20
“If” your rifle is a 3 1/4”, which is unlikely, then your single choice is Bertram brass from Buffalo at about $107/20
A budget alternative is to find once fired .470 brass and neck up to .500 then trim to length....same basic case.

I would wait on the bullet mould until you can slug the bores.....tho a soft alloy paper patch bullet and BP can easily deal with a lot of different bore diameters. It helps to already be familiar with paper patching tho.......

Graf’s has 1 set of .500 NE dies left in stock.

Unless the bore is severely undersized you can just order some 350 grain .510” cast bullets lubed with SPG and try ~136 grains of Swiss 1 1/2 or Olde Eynsford black powder (card wad and grease cookie required), or for NfB smokeless try 400-450grain .510” and appropriate charge of H4198 with Dacron, kapok, or backer rod filler & load it up.
Woodleigh makes 440grain .510” jacketed bullets especially for BP Express rifles.

As Huvius mentioned a simple 1/2” dowell, plastic tube, or even rolled paper, maybe wrapped in tape to a bit bigger diameter, can easily be slipped into the chamber to nail down the case length.

Oh yes, the best source for learning reloading in’s and outs for BP Express rifles would be Graeme Wrights book “Shooting the British Double Rifle”. If there are no more copies available in the ‘States (do a Google/Amazon search) then I think John “NitroX” here may be able to get you one for a reasonable price.

- Mike