There are many vendors who sell these devices, such as here: http://enablingdevices.com/cat...
For the specific need you mention, here's a start:
My wife has Cerebral Palsy, with very poor fine motor control of her hands. She has two of these joysticks, both paid for with grants found for her from our local United Cerebral Palsy center (http://www.ucpsdtechcenter.org/).
It is an analog joystick - the same type used on power wheelchairs: the further you push the stick, the faster the cursor travels in that direction. It has buttons dedicated to single-click, double-click, and drag. It has an axis lock-out button which toggles cursor movement between horizontal-only, vertical-only, and any direction. It has a button which toggles cursor speed from very, very slow to very fast. All these functions are independent of PC hardware and drivers. It also comes with a selection of handles: large rubber ball, T-handle, and small knob.
There is also a trackball version with the same features, other than a very large ball intead of the stick.
It's USB and works with any OS. She has used it with Windows (3.11 - Win7), OS/2, OSX, and currently Linux (Fedora) without need of any device-specific drivers.
We also have a couple of USB and PS2 keyboards with keyguards which were also paid for with grant money.
When looking for assistive tech (AT) for the PC, you can pretty much ignore anything mainstream-PC-ish. There is a huge industry dedicated to AT for most all types of computer and communication devices for the disabled.
[Note: the following assumes you are in the USA. Other countries may have similar programs]
While much of it is pretty expensive, some of it is covered by health insurance. And as I noted above, there is often grant money available from many philanthropic entities (e.g. MDA, UCP, Easter Seals, Rotary Club, Big Brothers, etc.). Funding may also be available from various government programs for the disabled (e.g. Regional Center for the Developmentally Disabled, state Disabled Services, Dept. of Rehab, the VA, etc.). Also try checking your local colleges and universities for resources and referrals. Many of them have a Disabled Services office on campus.