I spend a lot of time searching for new products to bring to USA, and readers of this blog can sometimes get a heads-up on what's coming to our product line-up. We've found a supplier of high-quality, excellent value USB bus analyzers and generators: MQP Electronics based in UK. Their products are easy to use and are well priced - see www.mqp.com. We should have them in stock in a week or two.
On their website is a useful document for getting to grips with USB protocol "USB Made Simple". Take a look at http://www.usbmadesimple.co.uk/