Like Joe mentioned, dentists will often be willing to give you some old
burs. I don't have many carbide ones, but I do have a whole mess of
steel ones that a dentist didn't want to try to sort out after an
earthquake many years ago. I might be able to send you a few. Also, I
notice as I write this that Google has put some suggestions for dental
bur sources in the column adjacent to this... you might try these, or
a Google search for dental burs:

www.net32.com, www.dollardiamonds.net, www.hobby-store.us

Also, if you go to your dentist armed with knowledge of how much the
burs cost (see net32.com, for example--looks like the best of those
three sites for what you want), you might be able to get him/her to
sell you some new ones at a reasonable price.

Cheers,
Tom