This was from http://www.scubaboard.com/community/threads/o-rings-for-suunto-transmitter.529183/
O-ring sizes for the Suunto computer and transmitter based on the measurements I took and successful replacement/use:
Computer battery cover for Suunto Gekko, Zoop, Vyper, Vyper 2, Vyper Air, Helo2 and most probably the new Zoop Novo and Vyper Novo: O-ring size = 026 (70 durometer/shore), Battery = CR 2450, 3v
Battery compartment for Suunto wireless transmitter: O-ring size = 023 (70 durometer/shore), Battery = CR 1/2 AA, 3v
(this is also the o-ring size and battery used on older Suunto computer battery compartments such as the Octopus-2)
Wireless transmitter to 1st stage interface: O-ring size = 012, (90 durometer/shore)
I sourced my o-rings via Amazon.com at roughly $2-$4 for bags of 100 in each size….free shipping with Amazon prime.
Hope this info helps someone else in the future.
After further research, I have found that the correct o-ring for the transmitter cover is a metric size 2mm x 25mm.
While the standard size 023 o-ring does not meet the exact manufacturer specifications, the one I installed is maintaining my transmitter watertight just fine after 10 dives with it.
Another recommendation I came across was to replace the transmitter cover when replacing the batter. Official kits come with new battery, o-ring, and transmitter cover. From what I have seen, these kits range in price between $20 and $60. Th reason to change the cover is that the 4 self-tapping screws might not create enough compression to keep the unit watertight after a few cycles of loosening and tightening, increasing the chance of the unit flooding.