Your LBS can really help here and I don't think you will spend more than if you went mail-order. There are some flashers, are you sure yours isn't one of them, that will bolt directly to the tab you mentioned coming off the rear of your rack. There will be a plastic nubbin on the back of the flasher which goes into one of the two holes and the screw threads into the other hole. This is to keep the flasher from rotating if the main mounting screw gets loose.
Planet Bike rack mount hardware is for Planet Bike blinkies, it will work with some others. Best, cheapest flasher I know of is by ViewPoint. I get mine at Performance Bike. All the cool light patterns but I never use them. Just standard flash for me. I'm so boring.
H