Harbor freight sells them as well.
I prefer to use good drill bits, not Hobo Freight junk, and wouldn't dare put a HF drill bit in a good barrel. You want to avoid creating a burr on the inside, and starting with a good sharp drill bit that cuts well is pretty important.
Most of their equipment of this nature is only adequate for woodworking anyway. It's getting hard to find good gear on Amazon too, unless you really know what brand you're looking for; there is so much low quality chinese junk being pushed there that it's hard to find the good stuff sometimes.
High quality drill bits are not very expensive, and are easily available from reputable places. McMaster, Fastenal, and Brownells are my main choices, MSC and Grainger if I have to.
I do a lot of metal machining and have learned a long time ago to not waste money on junk drill bits.
Edit - figured I should clarify the recommendations here: there are a bunch of good brands of drill bits, so it's not worth listing them all. Easy choice is to order them from McMaster Carr; they don't sell junk and if you order the right bit for the job, it'll do what you need. A 135° split point HSS stub drill is what I prefer, don't want carbide for this. A tip to reduce the amount of burr in the hole is to drill a couple number sizes smaller first, then finish with the correct size only removing about 5-6 thousandths. Also, for drilling gas port holes I only use the final size drill once or twice, then set it aside for other jobs. This has given good results for me in a bunch of SS and CM barrels.
Also, if you're using a drill press instead of a mill, put a cleaning rod in the barrel before drilling, it'll stop the drill from touching the far side of the barrel.
