You can try this link. They have a good inventory and I have purchased from them very easy to deal with.

http://www.pplmotorhomes.com/parts/rv-pumps-water-filters-fixtures-1.htm

Here is what you seek.

http://www.jazzsales.com/product_display.php?cca=38~44

Ebay is currently listing a combo unit with a 13gal holding tank for a toilet for $210.00.  With shipping of $39, it's the best deal in town.  In the search window ask for RV Shower and it will bring you to a few pages and you'll find the shower.  We are currently in the process of building our own LQ and have found a tremendous savings on ebay for new items.  Furnace, Hot water heater, Water tank, leather dinette cushins (haven't been able to find that deal again). 

Good luck 

Call this number these people can get you what you need 574-264-3882 and ask for jason.

I was having problems finding a shower too between fit and cost and ended up having one made by a friend's husband.

He measured the corner and made a stainless steel base to fit the corner and had the sides go up at least 6 inches.  Then he put the walls over that made out of waterproof material and made a drain. 

In the corner had been a closet so I already had the wall enclosed with a door and just use a shower curtain over the door.  I also already had a light overhead.

He also made me a small closet that shared the wall of the shower and it finished off the other end of the slant wall the shower is on.  

It works great for me and was half the cost including labor from what just a corner unit would have cost me.