I'm setting up a 3 way and mentioned the possibility of using female condoms. One girl said she asked for them in her favorite sex store and was told they were no longer available. Is this true?

They can be bought online as well as at the pleasure chest . Also a new FC is being designed by a new company as well right now , Pollie

I highly recommend The Pleasure Chest as well.  There are two locations, one in Midtown East / UES, and one in the West Village.  The east side location has a much better selection, but both stores carry Female Condoms.

I get them on Amazon or at Sinclair.com
They're actually cheaper there than when I purchased at Pleasure chest.
They sell out quickly in most stores. I've seen them once at a Times Square Walgreens out of all places. Day after , gone!

Yes, female condoms are tough to find in stores.  
A seedy, dusty sex store by me sells them individually for $5 each, but I don't trust that store with my safety.

I find them at Babeland in NYC and they have them online as well. I used one for the first time recently and it kept slipping inside of me. Do you have a way to keep it from doing that? Am I doing something wrong?

Now that I figured out how to use them...they are my preference and I always have a stock of them.

Then again...