eBay for the Behr matrix, or
autodoc et al. Don't worry about fakes as long as you buy from a reputable company like parts
Its not essential to drain your coolant but it is advisable. It's not a hard thing to do either, just undo the drain bolt with a suitable drain pan beneath it and the expansion tank off. Don't necessarily even need to jack the car up. If you don't drain it there's a risk that if your clamps aren't completely sealing the heater hoses (or if one slips) that you'll lose your entire coolant reservoir into the passenger footwell. That is not fun to have to deal with, believe me I know.