Your best option is to get an old, manual focus lens. While new lenses are available, they are likely to cost more with little benefit for your purpose (experimentation with lens reversal).
The mount doesn't matter because you will be using it with a reversal ring. If you want a lens you can use normally on your camera with an adapter, look for PK, OM, M42, or Nikon F mount lenses. If you're willing to spend a bit more, consider Pentax SMC lenses. Also, consider other focal lengths. 50mm tends to cost less.
Go to your favorite auction site.
Search for "lens reversal ring EF". Order 52mm and 58mm rings. (~$10)
Search for "28mm lens". Sort by price. Show "Buy It Now". Click on any that are of interest. Read descriptions to ensure there are no major problems. Try to find one that uses 52mm or 58mm filters. (~$25)
You may need a step down ring from 58mm to 55mm, depending on the lens you chose.
You may need a mount adapter, if you want to use the lens "normally".
Some lenses might be a bit more difficult to use.
- EF/EF-S lenses. You're likely already avoiding these because they don't have aperture ring.
FD lenses. Aperture doesn't work unless certain mechanism are engaged. FL lenses should be okay though.
M42 lenses that are auto only. If you intend to also use them normally, the M42-EF adapter will engage the pin to keep the lens in stop-down mode.
PK/OM/Nikon-F lenses. The aperture won't close unless a linkage pin is engaged. If you intend to also use them normally, the adapter will keep the pin engaged so that the aperture will work in stop-down mode.
Other mounts may have similar aperture issues. On most, you should be able to engage the aperture pin with a spacer cut from cardboard.
