Thanks for reaching out. I'm so sorry you're not able to find muscadine vines that can be shipped to Oregon. (I lived in the SE for a while and loved that unique muscadine flavor, too! Yum!).

There is a quarantine on grapes and grape vines coming into Oregon to help keep serious diseases and insects out of the state. (Not light reading but the specific rule can be found under https://secure.sos.state.or.us/oard/displayDivisionRules.action?selectedDivision=2729 scroll down to Quarantine: Grape Pests and Diseases 603-052-005.) Basically, this rule means that all grape vines coming in to Oregon need to be grown in a way that limits the harmful disease and insects that hitchhike on the vines. 

And you also nailed the other reason, the market for muscadines is very, very small here. In Oregon, backyard growers are focused on other table and wine grape varieties. So the folks at the muscadine nurseries aren't getting their plant material certified to ship to Oregon because the cost of certification outweighs the possible income. 

Have you tried reaching out to the Home Orchard Education Center in Clackamas? They may have a lead on someone already growing muscadines that may allow you to take a cutting from to propagate. (This group replaces the Home Orchard Society which closed in December.)

Best of luck!