Actually, you can get one for a lot less if you a DIY type.

I made two sloshers - one that holds 6 sheets of flim and that fits into an 11x14" tray, and another that holds two sheets and fits into an 8x10 tray. Bought some 1/4" plexiglass at Home Despot. Cut the plexi using my table saw and a carbide blade, sanded the rough edges to eliminate serious burrs, and then assembled them using plexiglass glue. Be sure to wear glasses or safety goggles - the saw will throw lots of small bits of plexi around.

You probably won't find plexiglass glue at the home center - I found it at a shop that specializes in glass and glazing products. You will need to get an applicator - a small polyethelene plastic bottle with a top that has a hollow stainless steel stylus. To use, you place the plexi components in position, and then use the applicator to inject a drop of the adhesive at the point where the pieces join - capillary action will draw the adhesive into the joint where it softens the plexi, and then almost immediate fuses the components together. The bottle of plexi glue will cost almost as much as a sheet of plexi, yet you will only use a tiny fraction of that bottle to glue together all of the components of the slosher.