Your service provider can give you one for between 50 - 100 bucks, but the problem is that it takes at least a day. Instead, just head down to any cell phone store and they'll cut your SIM for you faster that you can say Micro SIM
Really... it's not rocket science. They just put it in the cutter and press hard and SNAP, it's done. I decided to get mine done from a phone accessories shop as opposed to getting a cuter myself simply because they do 100's of these a day and they know which cutters are the best. Beats experimenting with el-cheapo cutters online and ruining your SIM card especially when you have a new device in hand and can't wait to get your SIM card in it. Besides how many SIM cards does anyone really have to cut? The guy charged me 10 bucks for it.
Its pretty easy. Took me less than 2 minutes. Google for the microSIM layout and paste it over the sim. Cut the sim's outer edges using a knofe/scissor and you are done.