TWO GOOD PLACES!
eBay: cheap but you can still find some good quality slim protective leather cases like this one:
Leather Case Pouch for Blackberry 8300 8310 8320 Curve - (eBay item 190179522326 end time Dec-06-07 12:31:23 PST)
Look for brands like Monaco and Milante or even SEIDIO on eBay in the $9.00 range.
SEIDIO: expensive and they don't do PayPal, but they still have the best range of good quality clip holsters and skins. Plus they have decent customer servie and a good refund/return/exchange policy-from what I hear.
As for everything else:
You can pay upwards to a $100 for a bombproof quality case made in some exotic place like Italy or Spain with Alligator leather but a horrible functional design OR you can pay 0.99 cents on eBay for a glossy cheapo plastic case that looks slim and nice but falls apart on you and even worse damage your berry. So remember stay with in your budget ($10-$30 will give you a decent case, more than that you are wasting your money) and ask around for feedback and reviews before buying.