We got the smallest one we could find for travelling with, which is also the cheapest - $20 from toys r us. Its so cheap I can't even FIND a brand on it. It is brilliant, you can just stick it under your arm and it takes up almost no boot space. Its a bit short though so useless for long walks, but we don't use it very often so we can cope with that.

Never had any problems with it, wouldn't be without it.