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

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

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