Try this outlet:

Lab Safety

They have a large selection of disposable respirators. If you are worried about toxic air pollutants, then I'd suggest one of the masks with two separate bands. The ones with one band are for non-toxic dusts only. These are particulate filtering masks, and are designed to be disposable. Use them and throw them away. The 3M N95 does a good job, and are not too expensive. This mask is also used by health care workers for SARS outbreaks. Here's the 3M N95 three-panel mask

There are others that use cartridges, which will filter very well but which need replacement cartridges. Here's the North Half-Face respirator

You can buy from the mail order bicycle outlets, but these masks are not rated for their protection by NIOSH. Therefore, you really do not know if they are effective or not. The masks above, available from any safety supply store (there should be one locally) will give you that level of protection.

John