I found a USA site to order the magnets from:
https://www.kjmagnetics.com/
They sell them in SAE sizes, so, 1/8" instead of 3mm. I ordered a 3/16" length, and some 1/4" and 3/8" discs to perhaps use instead of the buttons - I figure that would give an advantage when trying to glue things together, because it will hold them stable until they dry. OTOH, the buttons you used are pretty and look well with the cover. While I wait for the magnets to be delivered, I will also see what buttons I can find, and make a decision whether to use buttons or magnets when the time comes to put it all together.