Whatever your problem is, an outside air supply won't help. There is a lot of new research out that shows that they don't make a bit of difference. If you have a problem with your chimney etc. then you need to address the actual problem and not try a bandaid that doesn't fix the problem. See: http://www.woodheat.org/outdoorair.htm for some important info that better explains this. Many municipalities are removing this outdoor air supply requirement from their codes now. In certain circumstances flames can actually go out the air supply and cause a fire (this is rare).
I'm not an expert, I have just done enough research to realize that most people and contractors you talk to (including myself until recently) are lacking sufficient know-how when it comes to chimneys. You need to find a local expert and try to get at the root of your problem. You could also try to post some more details of your current situation and us board members might be able to suggest a fix. Our ability to help is usually directly proportional to how much detail you give us.
General chimney links: http://www.nrcan.gc.ca/es/erb/reed/wood/07_e.html http://www.woodheat.org/chimneys.htm http://www.woodheat.org/evilchim.htm http://homerepair.about.com/homegarden/homerepair/library/blchimny.htm http://www.jlconline.com/jlc/archive/energy/fireplaces/index.html
For makeup air for larger heating appliances sometimes you might need some makup air. See: http://www.jlconline.com/jlc/archive/energy/combustion_equipment/