I will need to get a re-entry permit on my non imm B visa (single entry) for some business trips, can i get a multi re entry permit???

TIA

Single 1,000 baht; multi 3,800 baht. Only valid until date of your extension so if you plan more than several trips during that period it is probably cost/time effective to obtain the multi re-entry type.

Can this be done at the airport or do i have to go to my local immigration??

TIA

You can do at airport only after you have checked in for flight so time might no be on your side. I would do at an immigration office myself.

Same to me, I prefer to have it in my passport before leaving for the airport.

Should you decide airport, don't forget to bring pictures, you wont be able to make them there.