I'm going to Myanmar tomorrow and I don't have the re-entry permit. Can I do it in the airport? My flight is at 11 am

You can certainly do it at Swampy and Don Mueang. IIRC both desks are 24 hours.

 

Note that at Swampy you get the permit before immigration, at DMK the desks are after (tell the officer you need a re-entry permit when he stamps you out).

 

Costs are the same as immigration, but (certainly at Swampy) there's a service charge of 100 Baht or so. Make sure you have photos, they may or may not be needed, but always best to be safe.