Nothing on the budget side. You might consider a converter lens if you have a 14-42 zoom. Olympus makes the WCON-PO1 for their zooms, and Panasonic has the DMW-GWC1 for their H-PS14042 zoom and 14mm prime. I use the DMW-GWC1 on the 14mm and am happy with the resulting 11mm results,
There are third party converter lenses for a lot less money, with probably a lot less quality. I found an old .45X converter lens from my Camcorder. It's very heavy though and would only work on lenses that don't rotate the filter threads. Can't use it on my 20mm for example, but I see it fits on my 14mm, so I'll give it a test soon.
I do not see the merits of defishing my Rokinon 7.5. First, it helps to frame the shot with this purpose in mind. Next, I think it's a lot of work to use the (free) hugin software, Instead, I would suggest making panoramas with hugin, which is easier. If you don't get too aggressive, maybe a two image merge with a 25% overlap, the software matches the images quite well. This is a a free solution while you save up for a WA lens.