Unfortunately, you just can't buy some of them and they're really rare on seas too.
There're also many models which just don't work.
For me, it was hard to find 30 and 36 Gun Frigate. I found them on seas after many hours of checking each formation. In case of 36 Gun Frigate I haven't met any which could have 24lbs + cannons. 30 Gun Frigate seems to be the best made model although I really like 36 Gun Frigate too (but board is weird) and Patroulle Frigate (in some version known as Raa Frigate. 30 Gun Frigate seems to be remodeled HMS Surprise and sometimes Surprise appears, looks to be higher but there're some problems with cannon and every time camera starts at bottom board.
I would like to see more corvettes and frigates.