I have in hand a 3 month non o visa for marriage , a single entry

for work i have to fly out from thailand for 1 week and I want to fly back with still have in hand this visa

Can i buy 1 re entry permit in the airport before fly out or can't I ?

Yes you can.

It will not add time but will keep your current permission to stay alive.

I have in hand a 3 month non o visa for marriage , a single entry

for work i have to fly out from thailand for 1 week and I want to fly back with still have in hand this visa

Can i buy 1 re entry permit in the airport before fly out or can't I ?

What you can do, if convenient to you, is to get first a 60 days extension for Bt. 1,900 at immigration. That is with reduced paperwork, just marriage certificate and wife ID and house book copies. After you have it, get the re-entry permit also at immigration. This way you will be able to stay and travel for two more months.