You can get prowlers by capturing the space marines during the quest "Taking Down the Hulk". The Hulk himself is a generic prowler too.
The total you can get from there is 5, I never tried to capture any in the The Prowler in the Depths storyline, but since alexsis987 said they caught the one in Temple of doom I would assume those are capturable as well (I haven't tried that either). I also believe there was another quest (one of the slave's stories) where you went to a research facility and there was an option to kill a prowler or add it to the household.
From the looks of them the only use for them is either donating to Fort Sera or selling them to Hunt Research, iirc the price was 7,500 $ per prowler.