Thank you for the excellent suggestion. I went to http://www.walgreens.com and found the right product (I think). I had noticed in the past that it was hard to find an enema bag that has an open top (so you can add more liquid while you're still attached, so to speak). The bags I had seen were closed at the top -- part of a water bottle combo. I think the "fountain" model has an open top.
Walgreens has both types -- for $11.99 each. And the website store locator found a nearby store. So I may be good to go now. Thanks!