Wilfie,
I would start by trying to find a local fish club or aquariam society. I will envy you if you find one. I am a member of 2 clubs, but the closest one I belong to is 5 hours away! Here's a link to get you started:
http://www.fishlinkcentral.com/links...merica/Canada/
Also try by searching using Aquarium Club or Aquarium societies as the keywords on google or whatever search engine you use. There are other sites out there that may have more clubs listed.
Once you have checked out the local clubs, I would join them, and get to know the other members. So far, everyone I've met in my clubs are great folks and they are more than willing to share stuff they have or sell stuff to you at a much better price than buying from a store. I will keep my eyes open for you, and give you leads to any I see for sale. There is also a specialty site which folks post mainly about all the shelldwellers, not just the multies. Their classified section might be a good place to look also:
http://www.shelldwellers.com
P.S. If you were willing to take a road trip, the above site folks just rave about the quality of the fish, and the availability of shelldwellers at this site:
http://www.wetthumbaquatics.com/