There was an east European company that did a plastic kit of a Viking longship in 25mm, might have been called something like Pola (but I doubt it 'cause I know they make model railway stuff), but they also did a pirate ship (worked out at about 15mm and looked like an old Revell kit re-packaged) and a Roman bireme (looked very like the old Aurora kit, perfect for 15mm but worked with 20mm).
One of the "model kit supplying traders that you see at wargames shows" (at least on the northern circuit) used to occasionally carry them.
Last time I saw them they were about £6 - £8 apiece.
If you're lucky someone might recognise my delirious reminiscences and point you in a more accurate direction.