Shimano only made thumb shifters for 6 and 7 speed. I used Suntour XC Pro thumb shifters with the "ghost click" on my 8 speed drivetrains for many years. I think there was also a work around with some after-market clamps and barcon shifters. If you don't mind going to under the bar STI shifters, you'd have better luck. You'll be looking for the M737 stuff.
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