The sea of claws is a probably a better place to ask this question (it's the most active Man O'War community that I know of).
But off the top of my head:
Demonworld games had some ship miniatures for their Empires Game. The 15mm ones are being re-released by Ral Partha Europe, so you might be able to find them on their site. Scale-wise, these tended to be a bit on the small side.
Also Green Forest Trading Company used to make dedicated proxies for Man O'War (mostly for fleets that were never officially released by Gamesworkshop ... I have his Lizardman fleet). Last I heard he was out of the business, but his website says he's still taking orders.
Hope that helps,