For seals, try Rock Auto. They had complete transmission gasket sets on closeout about 2 weeks ago. I bought two sets for $4.00 apiece and I think they have some left. I would think that a full transmission gasket set would include seals, but at that price I would take a chance. My set was for '49-'53 Ford. You didn't specify what you're working on, but I bet all side shift seals from that period are probably the same. Kind of a gamble, but I've bet four bucks on a lot longer odds; worst case is you will still have the gaskets.
Just opened one of the transmission gasket sets; sorry, no seals. Looks like $4 worth of gaskets though and they are Fel-Pro.