Not sure if Bumblings still have them, try the door latch section ? I got a stack of four button magnets approx 10mm Dia and 4mm thick each, Sorry cant remember the price ..
They are very strong [ currently using them to hold the door of the camper closed instead of a latch lock ] ..
Jaycar are expensive for magnets too .
When yo glue them onto the ally, shrink a piece of electric shrink wrap over them and the strip to help keep them in place