Polkadot and Kusama are decentralized networks, so any exchange or project can choose to list the DOT and KSM token.

Public listing services like Coingecko are good places to stay up to date on where DOT and KSM are being traded and can be purchased. Whether those exchanges serve your jurisdiction is something you can find out from their Customer Support directly.

Neither DOT nor KSM can be purchased from Web3 Foundation directly.