Yamaha DTXpress/DTXplorer/DTXtreme

You will have to look in the usual places - local music shops, ebay, Gumtree ...  However, a DTXpress IV Special kit won't be cheap.  If money is limited then you might have to set your sights lower - a DTXpress I, DTXplorer for example.  Also, buying a module and building up a kit from parts you buy separately may be cheaper.  Or turn your acoustic in to an edrum.  There are plenty of people who do that.  Look at 

http://vdrums.com/forum/forumdisplay.php?f=48
http://www.toontrack.com/edrum_for_free.asp

Keith.