Something akin to the ditto/mew glitch from generation 1. I don't have access to G/S or another game boy and would like to complete my crystal pokedex
Unfortunately, there are no recorded glitches in Pokemon Crystal that allow you to obtain Pokemon that are unavailable in the game itself. This includes the 2 other starter Pokemon you did not choose, as well as version-exclusive Pokemon to Gold and Silver. In order to complete the Dex, you would need additional copies of Gold and Silver along with another GB. Your best bet in obtaining all of the Pokemon in the Crystal Dex would be to ask around to see if anyone you know has access to any of those items.
Sorry to resurrect a dead thread, but the first answer is not quite right. There are methods to exploit Pokemon crystal into giving you Pokemon you couldn't normally get. Reference THIS LINK for more info regarding the Celebi Egg Glitch, as well as other glitches for unobtainable Pokemon in Crystal Version.