I've had good luck with BORMAX bits, and I think Woodcraft sells them individually. At least I have bought a few sizes I needed there. Rockler also sells their own brand individually and they have been good for the price. I started with a Rockler set and filled in a few gaps with Bormax. A few of the Rocklers that I could no longer sharpen I just replaced with new Rocklers or Bormax depending on where I was when buying. I've bought a few from Harbor Freight but not impressed, you get what you pay for and they were cheap.
I drill a lot of 1 1/16" - 1 7/8" holes in various hardwoods, end grain, 6-10" deep for kaleidoscopes and this is pretty tough on any bit. I have switched to some bits from MCLS that are more like drill bits for most of my k'scops sizes and for what I do they really are better than my best forstner-style bits. At least that is for deep end grain, side grain a regular forstner seems to work beter with more control.