I think it will be quite limited what you can find in resort - we only came across one small supermarket but there is a bigger one apparently. I'd go well armed if it was me!
I'm not sure because I have no childs, but normally you can find stuff as this in the Bonjour super market in the Gondola station and things as Pampas you can get in the Pharmacies, because it is not unusal in Bulgaria to sell these things there, but maybe ask again anyone who stay there now..
Inna hope you pick this up, the chemists (there is one inside the rila complex) sell pampers in packs or individually. We took our daughter last year and she was the same age, she stayed in the creche at the rila, and they fed her during the day, we used the milk at breakfast and there was plenty of choice of cereals for her. We are lucky in that she will eat most things so when we ate out we asked for an extra plate and she ate the same as us. Though in some places they adore kids so much ...
Inna hope you pick this up, the chemists (there is one inside the rila complex) sell pampers in packs or individually. We took our daughter last year and she was the same age, she stayed in the creche at the rila, and they fed her during the day, we used the milk at breakfast and there was plenty of choice of cereals for her. We are lucky in that she will eat most things so when we ate out we asked for an extra plate and she ate the same as us. Though in some places they adore kids so much they were actually knocking up meals especially for her. Baby food and nappies etc whilst being available are quite expensive in Borovets, we took plenty of our own. Hope this helps