Just my opinion, but I don't think you want to use a regular baby play pen. They have mesh sides and I would think the pups would chew and make holes. Also, they can climb up the side and probably get out or fall and get hurt. I've been looking also and think the doggie play yard type that have 8 panels that you can configure would be the best solution. You don't have to use all the panels. If you don't have a lot of room, just make a small square.
JMO
I've been finding them (metal type) at all the pet stores.