Martin doesn't appear to offer 20DP 20deg PA gears in hubless versions. I've checked Browning/Emerson-EPT and they don't either. I have a few possible options:
SDP-SI offers 20DP 20deg spur gear stock. The down side is that you'll have to bore it yourself and cut it to width. The up side is that you can make the gears whatever widt you want. I used this stuff to fix a problem with stripping gears in our tranny this year.
Rush Gears appears to have what you need. I can't get prices, so you'll have to call and get a quote. If all those gears they list aren't stock, then it's quite possible that it'll be cheaper to just buy the martin gear and machine it down.