Sounds like we also need to utilize a "grow out" pond to get hard to source plants over the hump.

Can you dig a little pond right behind your shop or shed where it is easy to maintain and water your eelgrass?

How about an old plastic tote. Cut the top off and throw in some good soil and fill it with good green pond water?

After you get your eelgrass thriving there, you could have a long-term supply of cuttings to try and establish it in your ponds that could use it.