David, generally speaking mould suppliers design their GC moulds to use standard gas check sizes. You should have little problem using standard 45 cal gas checks for .45-70. I use and recommend Hornady, as do most reloaders I know, but other brands should also work. You should be able to purchase these at most firearm retailers that supply reloading equipment and supplies.
I should add that mould base and bullet base diameters can vary a little depending on the manufacturer and casting metal composition. So some gas checks may fit a little tight and some too loose, but in general I have found it is not a problem with the gas checks but with the mould or caster.
In most cases I have found the bullet base can be pressed into the gas checks by hand then resized.
But I have a couple .45-70 caliber RCBS GC moulds where the bullet base is a little small (diameter wise) and the gas check will not firmly lock on the base, even after resizing. To retify this I plan on lapping the base of the mould to sufficiently increase the diameter. Some reloaders routinely anneal their gas checks to soften them and eliminates the tendency of the gas check to spring back to its original diameter after passing through the resizing die. I may try this also.
If the bullet base is a little large you may have to use the lubrisizer to seat the gas check prior to resizing.
These are just some comments that may help out. Hopefully you will get some opinions from others also.
