Also be sure that when you clean the plastic bottles you don't scuff up the insides. Things that would love to consume your beer before you do can colonize such scratches in plastic and become very difficult to dislodge. That said you're very unlikely to have any problem if you scuff them up cleaning them right before you use them. So if you're rough with them, just find more glass before you brew again.
I'll second asking around and seeing if people you know are just recycling their bottles and if you can have them. I grew a decent collection of bottles for my brew by doing this and keeping my own empties. While you're asking, if people have 12 or 24 pack boxes, they're really nice to have for storing said bottles.
Also, don't overlook larger format bottles. If you can pick up 22 oz bottles you'll have about half the effort on bottling day and almost twice the brew when you open one. Only problem there is the beer seems to disappear faster.