Ive never played with SS rotifers, and now I only play with the L strain. IIRC the L strain is a little longer and thinner than the S strain, but the S strain is a fat (wide) little thing, so the size isnt all that much of a difference. The L strain is easy to come by, whereas the s and SS are a little more difficult. Gresham and the folks at Reeds know quite a bit about rotifers, so hopefully he will correct me if I am wrong
Most hobbyists wont mind sharing their rotifers with you so you can get a culture going. That way, if theirs ever crashes, they can hopefully get some back from you . Keeping rotifers fed is the hard part. I beleive that most use the frozen paste from Reeds mariculture to feed the rotifers with good success. I use a live phyto , and thayt can become either time consuming, or costly.
BTW, what are you feeding rotifers too?