Home Depot sells tap and drill bit as a package here, not too expensive and easier to get the right bit.
tap drill charts are all over the internet. some times if you are putting a 1/4 20 bolt in a place where strength is NOT critical drill a slightly over size hole. makes the taping easy. i think a #10 is called out. but use a 7/32 for easy taping. but you WILL lose some strength. do NOT do this when strength is needed.
I have some taps that have what drill size to use engraved on them