For Laos specifically you MUST pay for your visa in Baht. Unlike before Covid, it's very difficult to buy any foreign currency in Laos these days, due to government regulations. Moneychangers are not allowed to sell you foreign currency, not even at the land borders. At airports you may be able to purchase a small amount of foreign currency, by showing your passport and boarding pass, but that's it.
Banks might be able to sell you some currency as well, but may require proof of what you need it for. There may be other ways to acquire foreign currency, but it's rather tedious compared to how easy it is to purchase foreign currency with just your passport in Thailand.
Therefore, ensure you have the
*****
Baht in cash BEFORE departing for Laos and to be safe, bring more and keep this
*****
Baht separate from your spending money there. ATMs only dispense Kip (except for reportedly, one ATM up in the Golden Triangle SEZ, which dispenses both Baht and Yuan) but that's pretty far to go for a Thai Baht ATM withdrawal!