Another low cost option for pocket squares is the tie bar ($8). I strongly prefer cotton pocket squares over silk. Here's a pocket square that has been featured in GQ (solid white, hand rolled black border):
You can also make your own pocket squares by cutting them out of second hand shirts as describe at TheStyleBlogger.
Are pocket squares in our or out? GQ has been featuring them prominently the last few years. Esquire says they are out. My view is that pocket squares -- done correctly -- add a nice touch when you are not wearing a tie.