There are at least a half dozen different styles of post poppets. I don't know if the tiny seals are separately available for each style (or any style, for that matter - I never tried to replace just the sealing part).
Googling "pin lock poppet price" provides plenty of hits. This Amazon page shows lots of different poppets. If you can match yours to one shown you're in business.
fwiw, I really dislike universal poppets. I have a pair of them on my two line cleaners and 50% of the time I use them the o-ring rides partially off the metal plunger and wedges in the hole at the top of the post, pinned there by the plunger. Sometimes I can work the o-ring back in place, but sometimes I have to disassemble the post. As a result of that experience I'll never use one on an actual keg...