There are typo generators out there, but IMO they suck for the most part. What I do is just type the keyword over and over and see where I make mistakes, and also where your fingers could slip.
For example, XBOX could be mistyped very easily as xboz, xbos, xobx, etc. The z and s are fairly close on the keyboard to the x, and switching letters is a common mistake.
Look for keywords with high search volume. Wordtracker has a great report for high traffic keywords, Long Term and Short Term. Although, professional domainers have probably picked it clean.
Keyword Discovery even has a typo generator built in.