For rabbits you should get a metal pen as they can chew through plastic.
Dog x-pens work great for rabbits. You do want to get one that is tall enough to prevent the rabbit from jumping out. 30" would be the shortest I would go with an x-pen, but 36" is easier to find. Some rabbits can still jump out of that, so you may need a top cover or get a 48" one.
I know Petsmart has x-pens, not sure about Petco since there aren't any around here. You may want to see if they go on sale at some point as they can be a good time to buy one.
Locally owned stores can also be good. I do prefer local stores as I am supporting local people rather than a big company.
Also look at used ones. They tend to be cheaper than new and can still be in great condition. It can take some time to find the right one at the right price, but if you have time it can be worth it.