Ebay is a great source, but caveat emptor applies - kit that appears to be bargain may well be, but people often use it as a method of shifting old or unreliable gear, stolen gear and counterfeits!
Counterfeit Shure and Sennheiser mics can be sourced from the Chinese factories for crazy prices. Some are quite good, others are simply dreadful - thin, tinny and weedy! They come in convincing boxes, but the black much is a little 'stiff'. The instructions and guarantee cards are pretty good too. One of my regular suppliers offered me some Shure SM86 condensers - and I have some of these and quite like them. The sample price was $5! I don't sell counterfeits, but I was intrigued. So I added one to the order. When it arrived, first thing was it was too light, slightly the wrong shape, but when I plugged it into the system, it worked. This was actually a shock because the phantom power was off! Investigation inside showed a dynamic element! Absolute rubbish. However, some SM58s and 57s sound very similar. The problem is that until you try them you don't know. It's clear that thousands of people think they have a Shure mic, from ebay, but have a cheap copy instead.
Music shops are always more expensive, but handy. On-line gets you a genuine Shure much cheaper, but the choice is personal.