Will be leaving (by air) soon to get exit and entry stamp for my non imm b. I am going to Laos.So, can anyone explain the re entry permit rules. Can you get one at the airport,( where, how much, how long does it take to get etc.,) or do I have to go to Suan Plu.

This is my first trip OUT on my non imm visa.

If you are going to use your next entry on a Multiple or Double entry NON-B, you don't need a Re-entry permit, and shall not apply for one.

If you are on a yearly extension from Immigration, or have started the extension process, you must have a re-entry permit before leaving Thailand, and you can get that at Don Muang.

Explaining reentry permits

Sorry, should have said I have a multi entry Non Imm B Visa.

So I take it that I don't need any forms to fill in before I leave.

That is correct.  You can just exit/enter as many times as you like during the time of the visa.  Each entry you can stay up to 90 days (if they stamp you right).  You just exit giving them your exit portion of tm.6 and enter with a new tm.6 and customs form.  Put 90 days are period of stay and enter your visa information.