Had you considered building a tiled shower? There are ways to do this that are not all that hard and you can make it just the right size and compensate for any unlevel/flat/plumb issues that might make a stall installation tricky. Cost wise, if you do the labor, it can be less expensive. There are lots of tile that aren't very expensive, but you can go overboard and spend big bucks. Wedi, Kerdi, Hydroban, etc. are all systems that can build a shower well and quickly. My preference is Kerdi, but there is plenty of room for others. Check out www.schluter.com to see how a Kerdi shower is built - the videos are interesting and enlightening. If you go a tiled route, check out www.johnbridge.com. It's unlikely, unless you have an oversized doorway into the basement that you could use a one-piece shower stall. Those are usually brought into the house during construction and the walls built around them.