I don't know any websites where you can buy portable chaff
There is a Chinese site offering 1,000 tons of aluminium ribbon for sale! You will need to know the radar frequency being used to cut the ribbon to probably half wavelength.
Option 'B' is to find a wind farm and loiter over that with your transponder switched off. If NATS is to be believed (they are demanding transponders up to 10,000' over wind farms because they say primary radar does not work over them - Wash Area TMZ consultation [of course transponders are mandatory anyway over FL010]) you will become invisible.