Wreckdiversupply.com has rebuild kits, including seats, for H valves/manifolds...not sure what the specs are on those seats if you want just them, however. The singles are still reasonably priced ($7) IMO, but I think they've doubled the price of the manifold kits ($30) recently (would have to dig in the parts chest, but I think the ones I bought a couple years ago were $15).
Honestly, having run into similar issues with archaic valve seats in the vintage paintball world, I think one could just spec out the seat seal material itself/its dimensions, buy and punch a sheet of it, and replace that part within the metal seat. The difference, of course, that it's not a terribly big deal if my Vector's homebuilt valve seat comes apart in the middle of a game...the manifold could be a bit more exciting. At the very least, the radial valve stem seal would get a good testing.