Bare minimum spices, all can be bought on amazon:

• Turmeric powder
• Red chilli powder
• Cumin (whole and powdered)
• coriander powder (you can buy seeds and powder them)

Ghee is also an important component of Indian cooking, but you could use butter instead.

A lot of the whole spices used in Indian cooking are readily available.

• Cinnamon
• Clove
• Nutmeg
• Green cardamom
• Fennel

With these spices, you can make a lot of Indian dishes--dals, basic vegetable curries, paneer dishes, rice pilafs, biryanis and even meat curries, butter chicken, etc.