You will likely find that your debit cards will work find at most Lao ATMs. Your currency conversion rates will be based on your bank's rates. You may wish to compare banks to see what their currency conversion rates are if you have more than one account, because some bank accounts may have a higher rate premium or charge higher conversion fees than others.

You will likely have a difficult time getting Lao Kip (the Lao currency) in advance of your travels, but this is not an issue. You can always get Thai Baht or US Dollar in advance as both of these are commonly accepted within Laos, and then once in Laos get Lao Kip as needed.