Up for sale here is this power switch with knob and attached board, part GWS-185, from a Pioneer SR-303 reverb. It is in good condition and will work fine, though it may need a little Deoxit. The knob has some corrosion/minor damage on it, so you may want to replace it. This should work on the Pioneer SR-9 as well.
US shipping and handling is $6, and international is $20, no matter what quantity you buy. All my small parts are set up for combined shipping, so just use the "add to cart" feature and then check out all at once, and it will combine shipping. If you have any questions, please ask.
I have more SR-303 parts and other vintage electronics listed--feel free to take a look!