I think it really depends on your budget. Potential candidates that come to mind are the KBT Pure or Race, for instance. But these are in the price range of 100-120 $, not comparable at all to the price of a HHKB pro. The quality you get is also in a different league, from what I hear (haven't had a HHKB personally, but I can imagine that there's a big difference).
If you don't mind paying the price for another HHKB, then go for it. But if you want to try something else to see if a less expensive board will do the trick for you as well, you could try a Pure, for instance, and resell it if you don't like it.