Congrats on the success! The Little Mermaid stage manager is really nice for the record too...really has always been generous with writing a personal little note + fullfilling whatever requests!
And that's all that will really sign. Even if you stagedoor, you won't really get everyone, and the stagemanager (despite the TLM one being so kind!) probably wouldn't do anything about it, as I believe I've told you what they do. Some shows only have 3 leads sign, so atleast you got the main featured cast + some understudies I think.
The only true way to usually get an ENTIRE (as in everyone) cast signed poster/playbill/yada yada is usually through BC/EFA Organization, which from time to time they offer items like posters, props other stuff for donations ot that charity, usually around $60-$120 I've seen based on what show/where (national tours do it as well).
Also you should (because you live in NYC I'm jealous!!) try to go to the BROADWAY FLEE MARKET, and at the Little Mermaid stand they could be selling signed stuffed or some other things! I've always wanted to go but can never make it...it's coming up in a few Fridays. Most will also sign autographs as well.
Last year at the Curtains booth was Jill Paice (<3 my fav!), David Hyde Pierce, and I think Karen Ziemba + some ensemble people. PM me for more details!
It's a flee market, in which each show has a booth to raise money for BC/EFA Organization that's usually run by members of the cast or stage managers, etc.