Lucky for you, that is one of the more common Baby Pony Wear outfits. Depending on condition, you should be able to find the complete outfit for between $5-$15, including the pocket pal. You can often find the pal alone for $5-$10, depending on the condition and the seller. There are some ebay sellers who can price accessories higher than that, but those prices are about average.
It's also one of the more common outfits to find MOC. Expect to pay about $25 for it, less if you're lucky, more if you're not.
I *may* have a complete outfit for trade/sale. I have several baby pony outfits complete with pocket pals in my trade bin, but I can't remember off the top of my head if that's one of them. I can't access it right away though. I definitely have the dragon suit alone for trade/sale. If you can't find one within the next few days, feel free to contact me if you're still interested and I'll have a dig in my closet and see what I can find.