Then you are locked into Precision brand pads...which are good but not everyone's cup of tea.
What I would suggest is that you get some cheap slide calipers and just measure the outside of each keycup. Supply the pad supplier with those measurements, and have them do the math to supply you with the proper sizes.
I guess I don't quite understand something:
You are buying just pads...so my assumption is that you are doing the repad, because if you weren't...why not just let the tech supply the pads, right (?)
So if you are doing the repad, you need to disassemble the horn anyway, and pads can get to you w/i a week if you Priority ship.
So..rather than just doing some sorta other way, which is gonna be compromised on a few pads (I guarantee it)...why not just start taking the horn apart now and getting the measurements exact ?
(Unless it's your only horn and you absolutely cannot lose access to it for about 7 days....which you would have to do if you brought it to a tech, anyway...)