Have you tried Aquabid?
There are a lot of people selling shrimp, and offering a dozen or more varieties. RCS are the most common. Look under the 'invertebrates' category.
I'm tracking several right now, as I'm wanting some also. They're available in small groups, (10-20), up to groups of 100. The most common offering is groups of 15 or 20.
Of course, research it a bit. Some offer better deals than others, especially with shipping. USPS Priority seems to be fine for invertebrates, as long as they have a heat pack.