Cheap super-wide angle options for 5D

Ok, the widest I've got is a 35L on a 5d. I'm now considering getting a super-wide preferably 17-20mm range. I can go fisheye, 14mm or the 24mm range too.

I don't mind if its a prime or zoom. Third party lenses are OK.

Manual lenses might be an option too, as long as its not hard to focus. Ie maybe just shooting at infinity might do?

It will not get much use. Primary purpose is for occasional building/indoor shots to show mood and environment of events such as weddings. Plus, a couple of fun people shots per event.

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