TON can be purchased from centralized exchanges (CEX) or directly from decentralized exchanges (DEX).
You can purchase TON from the list of centralized exchanges listed below.
Instead of using centralized exchanges, you can directly swap your tokens for TON from decentralized exchanges based on an automated market maker.
Even though TON is an ERC20 token, it cannot be swapped directly via Uniswap(and other DEX) due to a security feature. Only Tokamak Network Swap supports direct swaps between TON and other tokens.
To use Uniswap(and other DEX) other than Tokamak Network Swap, you wrap TON to WTON first using Tokamak Network Swap and then swap.