Hey,
I know of a fish store called Premier Pets off of HWY 10 in Ramsey (I think) that I sold my multi colony to several years ago, they may still have them. They have a located in Rogers, too.
Also, you could try calling A World of Fish and/ or Something Fishy (both located in Richfield, MN). I have seen shellies at both locations over the years.
HTH!
~Skap