CL in my area has almost become a 'match making' service between tenants and private LLs, as well as larger complexes. I advertise properties on it, but also have used it to look for prospective tenants (many people from out of the area moving in use it).

Have you considered putting in your own "wanted" ad, and stipulate a 100% smoke free building or house? There's also kijiji.com (which is very much like CL).

Best suggestion I can think of would be to ask in city-data's Philly forum, their forums are very handy for finding specifics about an area, including what you're looking for.

Thanks for your suggestion, and I will ask in city-data's Philly forum. I have posted on craigslist and kijiji, and the responses I've gotten have been:
1. Want to share my place?
2. We have an apartment for rent and we only smoke outside. (I want to be able to go outside, too!)
3. I have a condo that's never been smoked in, but the neighbors smoke.

I have never gotten anyone who had an apartment in a house or building where no one there smoked. : (

Something like 18% of the population smokes and that number increases in lower age/income/education segments, which may be more likely rent. So short of new construction, it may be hard to find a place that's never been smoked in and perhaps even harder to find a smoke-free complex. I also think only a few states actually make it legal for a landlord to ban smoking.

Have you considered renting a small house? I don't know if you are old enough, but how about one of the older adult communities where the odds of finding smokers is smaller.

You are just going to have to keep looking. I once rented an apartment in a smoke free building--it was just 4 units and was the only rental property that the landlord had, so he was very careful about who he rented to.

Given that you are also sensitive to scented candles and incense and most landlords don't even think about those, I think your best bet is going to be a small landlord, not a big apartment complex. Perhaps half of a two family house where the people in the other half don't smoke. Or if the layout is right, the other people can smoke and it won't affect you at all--no shared entry ways, no shared heating ducts. Or maybe an in-law apartment where the homeowners don't smoke.

mike_kaiser's suggestion of renting a small house is something to consider. If you can't swing the rent all by yourself, you could get a non-smoking, non-candle burning roommate to help with that.

Siena said "I am a non-smoker who gets sick very quickly from tobacco smoke (and incense, and scented candles, etc.)."

No landlord will, or can, ban scented candles or air fresheners. Your only choice is to rent a stand alone house if you are this sensitive.