Try MEC they have a system that looks much like what you have, the last time we got a replacment it was about 6.00 I think.
If you post the diminsions of the one you have I will check the one I just got to see if it is the right size...I just checked opend it is 10 inches by about 8.5 inches, will there fore hold a map that is about 8.5 by 5 inches or so. Another way around this is to go to a marine canvas shop that makes awnings and tarps and stuff and get some window material you can easily make you own with a little bias tape and velcro. Takes no time and you will have a map case that will not yellow or rip and is nearly waterproof.