We've got a handy page on our website about accessibility options and help getting online for people with Parkinson's - http://www.parkinsons.org.uk/gettingonline
Though we can't recommend a specific mouse, an organisation called AbilityNet may be able to help you. The organisation aims to make computer technology available to people with disabilities. Their FREE helpline 0800 269 545 offers expert advice and information.
The AbilityNet website is http://www.abilitynet.org.uk/
There is also a discussion that was on our forum last year called "Are iPads helpful to people with Parkinson's?"
You can still see the thread at http://bit.ly/18l1qMx
Hope you find some of this useful.