Have you tried Hobbyking? Simply put “Hobbyking 3s 40c lipo“ in google and you will get options.
Theres some very good small 3s packs from GNB and Tattu R line that are used in race quads, both an 850 mah3s, which is about max size for minis and a 500 ish 4s. I don’t see much advantage of 4s in a mini but it’s there if you want it. Decent quad suppliers like RDQ will have those. Hobbyking does some similar ones, they are fine for planes too and their 40c or more ones are usually good.
You will want at least a 40c for sub 1000mah, since C rating is related to capacity. Lower rated packs will voltage crash much sooner in the discharge cycle, especially when flying hard. Small capacity packs are worse for this.