I've been using DSLR Controller for ages and I love it!
I got the usb host adapter (sometimes called an OTG - on the go adapter) from ebay, can't remember which seller but they're all the same. I'm 99% sure the HTC desire uses micro usb B the same as my phone so you could get this one for $3.89!
http://www.ebay.com.au/itm/USB-OTG-H...item43b7f68fbc