You can get them for fairly affordable prices off of eBay.
MAKE SURE, however, that the garments have the official Air Force tags on them. Otherwise, they are not genuine and are technically, not approved for wear.
I bought myself several uniform items off of eBay with no problems - ABU blouse, ABU pants, and even a service cap and ceremonial belt.
Problems with purchasing uniform items from non-official sources. I believe it is technically not allowed to purchase uniform items from a non-reputable source. There are rules against selling of issued uniform items (since you get issued them for free, and if you sell them later, it's essentially stealing). I'm not too certain about the rules regarding buying of uniform items though.
Don't be afraid to ask, and if you think that it might be wrong - it probably is.