NewEgg.com had a Zonet usb dongle for like $10 after rebates... was a great deal... but seems to be over... Its back to about $25 including shipping.
I use an Ambicomm BT2000C-USB (older model) on my work laptop that I purchased at Circuit City a couple years ago... and a BT2000C-USB (newer model) on my home pc that I purchased at Compusa for $29.99 a couple months ago.
There are actually a couple threads here about bluetooth dongles already, so you might also want to search.
i like it because compared to one my friend has, its WAY easy to use. plug and play (windows will install drivers for you) and worked right away with my e815. i got it, plugged it in, then paired with my phone. after it paired, it recognized it as "Standard Modem over Bluetooth" and i was able to easily add it for use with MPT.