mamianka
Ferrees? Or check on the NAPBIRT site for suppliers. If you have the skill to do this, I am surprised that you do not know of a source for equipment. This is not a DIY project - you do not make it clear whether you are an amateur, or a legit repair tech.

pappa Xmas
Sorry, there is a limit on the efficiency of magnetic dent balls. The horn doesn't have anywhere except right at bell that is large enough to use a magnetic set. The problem is that the ball has to be large enough, to have enough mass, for a strong magnet to grab with the needed force. Dent balls are great on Tubas and Baritones but for smaller instruments they don't work. Get a dent ball set ($150) and hammers and and a cable driver for the placement of the balls.