Hello and welcome to Tana,
There are several baby shops in Tana, mostly scattered around town.Â
The first shop you can try is Zazakely which is downtown on the hill in the Antaninarenina area near "Pharmacie Metropole" (on Ratsimilaho road, but nobody uses road names here).
Then there is another shop in the Analakely/Pavilion area (watch for pickpockets here) called "Itokiana". It has many reasonably priced clothes and baby accessories.
Beyond these two you can visit some of the Malls around Tana (Jumbo, La Cite, Tana Waterfront) and check out baby shops.
Formula can also be found in Pharmacies (such as Metropole) or supermarkets.
Best,
Shahbaz