I've been down this road with a number of English guns, very rewarding but 10 gauge poses special problems.
As mentioned, first you need to establish chamber length, the Brits made 10's in lengths from 2" up to 3 1/2" and more. The most common, almost standard length is 2 5/8" but dedicated wildfowling guns frequently had longer chambers, sometimes much longer.
2 7/8" which is more common in American guns will be encountered but may have been lengthened to this dimension ( with associated risks). Chamber length is measured with a simple chamber length gauge ( like a properly sized dowel ) which can be purchased but is frequently home made out of steel, brass, aluminum or hardwood on a lathe. It can also be measured with some shotgun bore diameter gauges if you can find someone who has one.
Suitable black powder hulls are hard come by in ten gauge but you seem to have that issue solved. The most commonly available paper cases are old obsolete Canuck and Maxum 2 7/8" ammunition from the 1960's or older, these can still sometimes be found at gun shows and they will be of very erratic condition. Many will be swollen or have cracked brass and should be rejected. The sound cartridges will need to be fired in a strong modern gun to provide you with the empty hulls to reload, don't fire them in your old black powder gun. Once empty they may need to be trimmed to match your chamber length, if your chamber is longer than your hulls they can be used with no alterations. Ten gauge brass cases are not commonly available at reasonable prices, and the ones I've found are usually 2 7/8" or shorter. Trim to fit your chamber if required.
To reload you have many directions available. For traditional black powder loads you will want fibre, cork or felt wads and a rolled crimp. This is a slow process but can produce beautiful ammunition. Modern tools can be used for all stages except the crimp stages and the old traditional hand crank tools still work well with the paper hulls. The original Lee Loader table top unit works well if you can find one. In addition, because your hulls will have been originally fired in a different gun or guns they may need to be resized, check all of your empties in both of your chambers to see if they fit. If they fit no sizing is required. Any primer will work fine, but I favour the hotter ones. Ten gauge black powder charges were generally 3 1/2 to 4 1/2 drams of FFG by volume. Use an over powder nitro card ward , build up your cushion wads to the correct height, then another card wad, then your shot charge, then your overshot card wad, then close with a roll crimp. Perfect! They will reload several times but the crimp area will break down and they will get pinhole burn throughs just above the brass and must then be discarded. Paper cases are not suitable for a modern type fold crimp.
Brass must be dealt with differently. First, if previously used with black powder they must be thoroughly cleaned of all fouling and corrosion and carefully checked for integrity. Modern primers may or may not fit. Sizing if required is much more difficult. Powder charge remains the same. Wads. Here is where the problems start. Brass cases are thinner and require bigger wads to fit properly, usually 9 gauge in a 10 gauge gun. These are nearly impossible to source in Canada, must be ordered from specialty suppliers in the US. Also, a roll crimp is no longer practical or desirable, you will need to glue your overshot card wad ( 9 gauge as well) in place with any basic white glue. Then you will probably want to slightly roll the case mouth inward to eliminate the sharp case mouth. You may need to make your own tool for this.
A simpler but non traditional method is to use Remington SP10 plastic wads in the paper hulls and roll crimp. They will not seal properly over the powder in brass cases. Feel free to PM me if you like. Have fun.
