I have this one:
North State Superyard XT Portable Playard and Gate - Walmart.com and I think you can get extra pieces, but be warned,
because of the mesh sides, they can learn to climb it which is what my dogs do, and it can pose some dangers in getting their feet/legs hung up, so I don't ever use it as a pen. If you have a climber I would not recommend this model. But it has become very handy to use it in pieces to block off rooms or areas of rooms I want to keep them away from.