Minimal order value on exchanges are likely $10 but you can check it with some websites. You need to check their minimal withdrawal and withdrawal fee too because at the end, after one or some purchases, you will have need to withdraw your bitcoin from exchanges to your own wallet.

You must know a minimum value of your bitcoins in the exchange account, and cost of withdrawal fee to avoid turning your money to dust after a withdrawal.

https://www.cryptowisser.com/exchanges/
https://exchangewar.info/
https://withdrawalfees.com/