There was an old one in a stack of rubbish that was probably salvageable at Ocean Marina next to Seacats Boatbuilder. However a couple weeks ago it disappeared, while the rest of the junk is still there. Would probably have been ideal for you if you fancy of fiberglass work.
Check at Sattahip Navy Sailing Club as well. Members of the public can go inside, just ask anyone you see around the grounds. There are dozens of older boats of many different classes there rotting away. Some probably can't be sold due to Thai bureaucracy, but some are definitely privately owned and worth asking about. https://goo.gl/maps/wR7vxvMzYN3mDDBt8
Baht-Sold sometimes has boats listed as well, good idea to check that once a week or so.
The ones at Varuna are very likely to be quite expensive, but it is worth checking.
Alternatively, ILCA Masters Worlds is coming up at Varuna in early 2023, so there will likely be a pile of ILCA boats for sale after that, but same thing, likely very nice boats and very expensive.
