My magnet was from the local hardware/ironmongers. (Manufacturer/distributor is
www.centurioneurope.co.uk but their website looks at first sight like it might be trade only..) Ferrite/ceramic disc magnets will not be powerful enough for this, but Nd will do the job. I found that the magnetism is your friend as very small adjustments can be made while setting up, without the booster shifting around of it's own volition. (i think i used blu tack to wipe away any debris between the mating surfaces)
I got lucky insofar as the size and power of the booster was ideal. Any larger would not have fitted, and much smaller might have required a sister magnet over the other side of the concentrator.