EAA has info on the web regarding application for the Repairman (not "Repairman's") Certificate. Go to this page, read the AC, and when you scroll all the way to the bottom you'll see links to the blank form (FAA Form 8610-2) as well as a sample form showing how to fill out the application.
As has been mentioned in this thread, DARs are not authorized to issue this certificate. Even FAA MIDO inspectors can't issue it. Since it is an "Airman Certificate" only FSDO personnel are allowed to issue. You must appear IN PERSON and show your photo ID in order to make the application. Any FSDO that is issuing these through the mail is not following the procedure called out in the field inspector's handbook.
It may not be necessary to travel all the way to the FSDO. Inspectors are out in the field quite often for various reasons, so you can arrange a meeting with a FSDO inspector when he/she visits your local area. Have your photo ID (driver's license or equivalent), your completed application, your airworthiness certificate and operating limitations, and your construction records. If the inspector does his/her job correctly they will inspect all your documents, ask you a few questions while reviewing your construction records, and assuming they are satisfied that you are the original primary builder of the aircraft they will issue the certificate.
As with all airman certificates, you'll get a temporary copy and the permanent certificate (in a plastic card nowadays, just like the new pilot certificates) will arrive in your mailbox in a few weeks.