Getting a waterpik is an excellent idea. Many years ago when my son had braces, he was absolutely terrible about keeping his mouth clean. I ended up trying a Waterpik, and the orthodontist noticed an immediate improvement.
I haven't bought one since, but at that time, nobody really had sales on them, and most places had pretty much the same price. What I would now do since you want to get one immediately, is buy one where the store will price match if you find a lower advertised price in the next few weeks.
Be advised though, that it took my son almost a week to learn how to use the waterpik without shooting water all over the bathroom. It is difficult to get used to, but for my son it was an absolute necessity.