Why wouldn't you buy a 1st hand third party lens or a 2nd hand Canon lens? :bsmilie:
For the Canon system, I think the Canon 10-22 receives very good reviews, but for third party UWAs, Tamron is usually not considered as they don't make good UWAs compared to the other third party makers. The popular third party UWAs are
Tokina 11-16 f2.8
Tokina 12-24 f4
Sigma 10-20 f4-5.6
Check out the review sites to decide what you want to get. You should take note of flare resistance because it's quite hard to avoid getting a huge light source in your photo, and distortion characteristics (if you want to shoot landscapes with straight lines and architecture, and level of distortion is not as crucial as the difficulty to remove them). I think these 3 have very good sharpness.