Baby Gusty is the only Play and Care Set baby (please, trying to get rid of people using 'NBBE' as a term for these ponies, as it's ugly :'() that I have never had a double of. I don't know why she's so tricky to find but she does come up occasionally, just it seems to be in patches.
She does come up sometimes on ebay but it's worth looking at non-US ebays. She's more likely to show up on ebay sites from the countries she was sold in (UK, Germany, etc).
Here is also a good place if you posted a wanted ad. Someone might have one they want to sell.
Also maybe try searching with the proper set name? You never know.