Can anyone tell me where I can find a mother board for a gateway computer

If Gateway no longer supplies the board, I usually just do a Google search for the board part number or system model and find a surplus source. If you call Gateway, they may give you a source, however I thought their prices were ridiculous any time I've contacted them ($300+ for mobo). Alternatively, a form factor compatible board from another vendor which works with the existing processor and memory can be installed and the operating system reinstalled.

Try eBay there's a bunch of them there

To simply replace a motherboard you will need to know a few things first,
-Socket type (1366 ? 775? AMD vs INTEL )
-Ram type ( DDR ? DDR2 or 3 ? )
-What hard drive format it supports ( IDE vs SATA )
-Possible peripherals and other (Video card)

This is to ensure all your pre-existing hardware
Will be compatible with the new motherboard.

As for where to buy , Newegg.ca , Canadacomputers
Or a place to compare prospective motherboards
Would be shopbot.ca .They list many online retailers
Competing for your business. Take a look , may save
You money.

This could also be handy to learn about motherboards to make a more informed decision
Intro to Motherboards