This is what I used but you'll go through about 4 or more so I suggest something more serious. There is a bit of a learning curve involved when using them so you don't blow them up, but once you get the hang of it you can get about one hole elongated per bit before they're toast.
455 Chain Saw Sharpening Stone / Model: 455
I'm thinking the back up cutter I bought but didn't use was one of these ones.
Tungsten Carbide Cutters
It needs to be able to hack steel. You can read what they'll cut in the description of each bit. Bits for soft metals won't survive, nor cut worth a s***. Using them will just piss you off, and you'll be under there all day cursing. Ask me how I know.
Oh, and Amazon usually has the Dremel bits real cheap if you can wait a few days to get them. I got the pair of 115s (or 117s) for a fraction of what they'd have cost me at my local Home Depot.
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