Yes Aristo's 21" and 31" diameter tracks are available as a complete circle, I have used several now for different aspects. Be advised they come in 4 quarters so track planning is a little more difficult but it can be done, you will be very limited in your rolling stock, smaller the better, but for those space challanged, these offer a really interest world of possibilities.
Wholesaletrains.com has them available, in fact I just ordered another set of both, call to see if they ship international, I beleive they will.
If you're looking for tight curves for narrow-gauge mines etc., then I think Modellbau Heyn maybe do some bits and pieces either as component parts or possibly as complete units too. Unfortunately I can't check the website as it's blocked here at work, but it should be: