Well I'll be the first to mention that a fly plaid is hardly required...

But you do have some options. First of all I believe the minimum order to get double width material from Dalgliesh is 16 yards (they may have upped it to 30) four of those yards will go into your kilt and two will make you a nice square plaid. So you could try and find 2-3 others who would want a kilt in the same material.

Failing that I would direct you to talk with Scotweb or Matt Newsome. One of them may be able to get a fly plaid woven to correct dimensions with their contacts.