There's a lot of great Kurt depictions on the internet, that's one of your best places to start. This site actually has a pretty got clump of artwork of the Elf, both fan and official creator. Dave had a commemorative art book come out sometime around his death (I believe), but I have no idea how much it is or where one would find it (or even what's in it).
There's Marvel/X-Men "How To Draw" books for kids that I've seen, but I've never see a Nightcrawler specific *art* book. I really think your best bet is websites.
If you hit deviantart there's definitely going to be some actual, current, Marvel (or even DC?) artists on there who have sketches of the characters they like to use, and I can almost assure you there's gonna be a half dozen Nightcrawlers once you search him.
R.I.P. Ultimate Peter Parker
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