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You could have a workplace assessment (assuming you are in a workplace where someone else supplies the equipment) if you feel very uncomfortable. The sort of problem you seem to be having should not be taken lightly, as you could develop RSI from it. As timmah!, hamrack, and dave4 have hinted, there is a great variety of mice you could try out. Your best bet is an assistive technology web-site like http://www.posturite.co.uk/ergonomic-mice-keyboards.html. Many of these offer trial periods (unlike the shops you've tried), such as 14 days at this one, so you might be able to find out how you get on with a device before keeping it. They're quite used to people returning things after a trial, so use that facility. Also, some universities have assistive technology centres, so if you ask very nicely you might be able to try something out there. Hope you find something that suits you--pretty much anything is better than a standard mouse.
Can anyone recommend me a decent, comfortable mouse? I do CAD all day (2 and 3D) and the standard Dell mouse is turning my hand into a claw.
Apparently shops don't have mice out to fiddle with any more.