If you are in the US, most major pharmacy chains such as CVS, Eckerd, Walgreens etc carry them, although they are sometimes not prominently displayed. Look for a combination hot water bottle, douche, and enema syringe. The cost is typically in the $10 - $15 range.
Or you can purchase the combination bag pictured at the link below by going to http://www.walgreens.com and putting the word enema into the search box. That combo sells for $11.99.