Any rare earth magnets will do. I got mine from an electronic supply storefront. Try craft stores too.
I used magnets on both the speaker baffle and the grill -- no a metal slugs. I also recessed them in the front baffle, glued into the hole, filled, sanded and painted (or veneered). You cannot see them when the grill is off. I love the look and the operation. I would strongly suggest every DIY speaker build use a magenta approach. ...just be careful of polarity... I wasn't the first time, HA!.