I confirm you can use any capacity Li-Ion battery that is the correct voltage. (I am an electronic technician.)
For some quads there are various capacity batteries available, but from experience they are not always the capacity claimed !
(You can check battery capacity when charging through a USB meter with mAh function.)
(But the laws of physics mean a bit less comes out than goes in.)
Banggood.com & Aliexpress.com have a lot of parts and stuff at low prices, but things can take a long time to arrive from China.
If you know someone with a 3D printer, they might enjoy making prop guards and small parts for you.