I know that you want to buy a professionally built incubator, but take my word for this...its quite easy and inexpensive to make a large high quality incubator.
I have never bred ball's but I have bred other species with this type of set up very successfuly.
get any kind of large aquarium like tank (im going to use a 40 gallon in this explination)
-cover the glass with double thickness black paper
-buy roughy 3 to 4 TEN GALLON sized heat pads
-place the heat pads at each corner of the tank
-put roughly 1.5 inches of substrate on the bottom of the housing
-place the eggs about 2 inches away from the sides of the tank (make sure they dont touch the glass)
-This will make the temp inside about 87-90 degrees (should be optimum for balls i believe)
-Place anything on the top of the tank to keep it nice and dark, and make sure its tight fitting (not air tight) so that heat does not dissapate
just an idea to try..has always worked well for me