When I bought my Sennheiser kit a few years ago, I bought a Sanken COS-11D lav mic on the recommendation of several friends who use them daily and I don't regret the purchase.
http://www.sanken-mic.com/en/product/product.cfm/5.1105900
It comes in black, gray, beige or white and has several accessories that you can buy for it.
I highly recommend buying all your gear from a professional audio shop such as Trew Audio, Location Sound Corp or Professional Sound Services just to name a few. Pro audio is all that these folks do and they will make recommendations based on your proposed usage.
When you buy the Sanken mic you have your choice of connector ends (XLR, 1/8", etc.). If you have any plans of hooking it up to another camcorder that uses an XLR plug, have the 18" plug installed and buy an Ambient EMP3.5 power adapter. This will convert the 1/8" plug to an XLR and is well worth the money as it adds to your flexibility.
http://www.ambient.de/en/products/ambient-recording/mic-power-supply/emp/emp35.html
Mike
