DSiPaint users can now use Savii bucks to purchase additional slots for saving Saviis. Everyone starts out with a limited set of ten default slots before being unable to store any more. You can increase the capacity by 10 with 10 Savii bucks. Slots are sold in no other increments. The Savii bucks page can be found from the Saviis menu on DSiPaint or here.
This has existed before. Due to credit card fraud two years ago, adding more slots and purchasing Savii bucks with real money were removed. But as promised, the feature has returned.
Earning bucks without the use of auctions can be done by playing Savii Slots, a game exclusive to DSiPaint and 3DSPaint. Do keep in mind that the chance of receiving the prize of 1 Savii buck is relatively low. See the stickied FAQ thread for further details regarding points and bucks.