Where can I buy ethereum tokens on a testnet like Kovan? I have tried going into regular exchanges (e.g. Binance) but it seems like they only provide functionality for mainnet.
When exchanges or other financial tools talk about ETH (Ether), they almost always mean the ETH on Ethereum's mainnet. This is not the same as testnet ETH in pretty much any way - they are completely incompatible with each other.
The general point of testnet ETH is that it is valueless. There have been some experiments in other ecosystems with "incentivized" testnets, as to say that the native testnet token has some value, but not as much as the mainnet token. (Kusama network may be the best example of this as of this writing.)
As of right now, all ETH testnets are valueless. There has been some talk about making Goerli's ETH have some value, but no serious effort has been made, and to my knowledge, the major validators are opposed. One of the reasons they remain valueless is due to things like faucets - sites that will gift an amount of the testnet ETH for free. The amounts vary from faucet to faucet (and some faucets have different "tiers"). Sometimes you can find validators/miners/etc who are willing to gift large amounts of the ETH for test purposes. (I know of one case of 1 million testnet ETH being gifted for testing purposes.) As mentioned in the comments to the OP's question, Kovan also has faucets, and the amounts provided should be enough to cover dapp testing in most cases.
Hope that helps!
You can use the - https://faucet.rinkeby.io/ for Rinkeby.
NOTE: Use a Twitter post option (had issues with Facebook post).
I'm late to this, but you can do a couple things to get testnet eth: