You've a choice of at least three sources, Joshua:
Although Ducati don't list them separately for the Evo, part no. 88641181A is listed as the fuel pipe o-ring for, among other models, the ST4, 748 and Monster 900, and works on the 696/796/1100 Monsters too.
The Triumph equivalent (T1240181) is identical and also works. Although it's black, it is viton, (I have the email from Triumph's technical department to prove it!).
Here's a comparison between the Ducati and Triumph rings (green - Ducati; black - Triumph).
Then there's the CCA o-rings that Darren linked to but they're about seven times the cost if the genuine Ducati items.
I've got some spares so, if you PM me your address, I'll pop a couple in the post tomorrow.