They have their own shipyards capable of building designs suitable for their purposes - they rely heavily on stealth to do their thing and the Raiders and the yards have to hide pretty well. Its not easy taking a full blown warship and expecting to keep it, so they tend to strip and sell (hides the evidence as well) - they keep systems they want.
There is a race that kind of fits what you want - the WYN - thats a cluster of pirates, outcasts and exiles based in a big nebula object on the corner of Kzinti, Klingon and Lyran space. The nebula totally knackers ships for a while after they pass through and the weaker ships of the WYN fleet then hit the bigger enemy ships while their shielding pants are down.
They have a range of converted civilian ships (auxiliaries basically), plus a few smaller ship from the surrounding races either nicked, interned after they pile into the nebula, defected after fleeing to the nebula, or sold to maintain the balance of power - they have significant contacts with the Orions and in later stages of the SFU a series of homegrown dedicated warships come into play. I'd suggest that they are what you are looking for in the SFU.