What is your budget? A mex fender neck is around £270.
Last one I bought was from Andertons I think.
Someone also has a Mighty Mite neck on ebay at the moment but it has several days to run.
If budget is less than £150 then the suggestion above is a fair one. I put one on a Bitsa around a year ago. It was OK for the money but I would probably look out for a used Fender Mex, Mighty Mite or Allparts in preference.