Walter, Norton & 3M all make a good high end product with great adhesives.
An option to the flap disc's are the old school Norton Norzon F981 or the 3m 982C and run them with a hard back up pad ( depending on your end use ). A 5'' back up is like 15.00 and the dics's run in the 4.50 range.We run these on a daily bases for years.
A flap disc is attached to a air cooled back up pad and you pay for the abrasive and the back up with every disc rather than just changing out a disc.. just my .02 ..