A custom will cost you a fortune. Vintage models are very hard to find, and are very costly too. If you're happy getting a Japanese reissue, there are a number of outlets you can try:
http://www.excite.co.jp/world/english/w ... N&wb_dis=2
I don't know how good your Japanese is, but the following site won't answer if you email in English. Ishsibashi can however act as a third party, for a fee:
http://www.taniguchi-gakki.com
Go to the above site, Click on the tab 'Fenda Japan'. Then go to the 5th tab on your left (Fenda Japan Ereki Guita II). There will be a large selection of lefty jags, jazzies mustangs and teles.