Where should it cost less to convert Thai Baht to Lao Kip?
In exchange shops in the depth of Bangkok or in an airport in Bangkok?
Travel Stack Exchange is a question and answer site for road warriors and seasoned travelers. It only takes a minute to sign up.Sign up to join this community
Assuming you're going to Laos, the answer is neither: you'll get the best rates buying Lao kip in Laos itself. The currency is minor, not really convertible, and unstable, so exchange rates outside Laos and maybe border towns will be terrible and many changers don't even offer it. Even in Laos, larger purchases including visa fees are often quoted and charged in USD, THB or CNY, depending on where in the country you are. If you're (say) crossing over from Thailand to Vientiane by the Friendship Bridge, you can pay your visa fees by USD and tuk-tuks, hotel etc easily by THB, meaning there's no need to change in advance.
If for whatever reason you absolutely have to buy kip before arrival, one of the major dedicated exchange chains (Superrich, Vasu, etc) will be your best bet and certainly better than the airport.
As @lambshaanxy says, you don't have to buy Kip. The THB is accepted just about everywhere, including at the border for the visa. Caveat emptor, though, exchange rates can be wonky, especially for a farang.
If you want/need Kip, the best exchange rate you'll probably get is from an ATM – depending on your bank's policy. Whether your card will work is a bit of a crapshoot though. I'm lucky to have cards with the UnionPay logo (the Chinese network), so they work at many ATMs in Laos.