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Zooplus is a good place to buy playpens at a good price, they do have a minimum order though so you would need to stock up on a few other things too, not so hard!
This metal one is good & easy to fold flat & this wooden one is a good alternative.
The stylish playpen CMB mentioned is a good option, very easy to make, you can make it almost any size you like & it's easy to fold up & put away.
I've also made one with sheets of correx, just cut four pieces to the size you want, make a small hole in each corner, join with cable ties & job done! That folds flat easily too.