>>3583702The Tamron 17-28mm is the best option for the price if you can afford it. The price is "up there" because it's high quality optics.
Otherwise if you're really super poor there is the Samyang AF 18mm. It's the better option than the Sigma crop lens.
Alternatively you can preorder the Tamron 20mm prime lens.
If you're okay with manual only lenses, Voigtlander has a bunch of super wide options, as well as normal wide options.
I noticed their 35mm F1,4 pancake is on sale.