I know this post is older but maybe you are still looking for an answer.
1) The care sheet for them says a 40 breeder is fine. With one of my lessers it isn't. Most sirens are sold as S. Intermedia so you could be getting a S. Intermedia Nettingi, which are larger. I have one that is around 4 years old and he is 21.5 inches long. He lives in a 75 gallon tank. Anything less, I think, would be small and cramped. I have another smaller one in a 45 gallon tank, but he/she is only a little over a year old, 12ish inches and not fully grown. I think it may not grow as large and is a different variety of lesser siren than my older one, guaging by the size, color, and age.
2) Backwater reptiles has them for sale right now.
I hope that helps.