I must say Fizzy is pretty awesome!
As far as retro stuff Fizzy isn't on that much stuff. She was on a pair of socks at Primark, but I'm not sure if they would still sell them. You can however find them on eBay. She was part of a 3 pack of socks along with Milky Way & a Starshine Unipeg?
Typo brand My Little Pony Pencil Case with TE ponies (not sure if it's licensed or not)
Pyramid International has a retro notebook coming soon with Fizzy & several other ponies on it.
Amazon UK shows a bedding set with several G1 ponies which may include Fizzy as it looks like the same pattern as the above notebook.
Pastel Dreams has an overpriced pair of platform shoes with her on (although she has the wrong colored hair), but it's good for a laugh at least.
Vintage stuff:
Fizzy has a few variants including 2 Spanish variants & a reverse symbol one.
Blue boardered folder with Fizzy Muchik & several other ponies.
Very Rare stoneware mug 1986 found in Sweden & Scandinavia? Also, a sipy cup & bowl may exist.
She was in several of the comics. Including #42 Fizzy & the Flying Fish; #57 Fizzy's Fizziwhizz Party; #168 FizzyWhizz Flower Night; #194 Happy
Fizziwizz.
Fizzy was in a lot of the original tv show which as been released on DVD. I don't know if that was something you & she watched as children, but you could get it & watch an episode that has her in it a lot. Or if you can find scanned images of the comics (again I don't know if you got those as children, but it would be something fun to share together.) I know there are scans of Applejack's Amazing Adventure (it tells the origin story of the Twinkle eye Ponies) that you could probably print out & share.
Unfortunately Fizzy really isn't on a lot of merch, but hopefully with any luck you'll be able to find a way to incorporate her into your sister's b-day.
Post Merge: October 26, 2017, 05:29:39 PMSomeone on Tumbler named Heck Yeah Pony Scans posted these from the comics & they are pretty amazing:
Applejack's Amazing Adventure
http://heckyeahponyscans.tumblr.com/post/63730194370Fizzy & the flying fish
http://heckyeahponyscans.tumblr.com/post/30922070856