Lowest mk level I see on the exchange is VII, captain. If you can find Mk V or VI, that'd be commander.
They might seem that way at first glance, but the proc is only 167 shield damage. Not a lot to write home about, and one of the main reasons a lot of people complain that tetryons need a buff (along with plasma and polaron).
If you want a way to do extra shield damage, I'd recommend antiproton or phaser. Phasers have a chance to shut shields off entirely - it's a lower chance than the tetryon damage proc (because the shutdown isn't always shields), but statistically you'd only expect 652 extra shield damage in the same number of shots you'd average per shield shutdown - though you can't always capitalize on a system shutdown, you always get the benefit of extra damage.
Antiproton are harder to come by - I don't think they show up at all until mk IX at rear admiral, and there's less sources of them than other weapons (crafting and STF token exchange), but their special is increased damage on critical hits - whether they're on shields or hull. You'll likely crit a LOT more often than 2.5% (especially if you use weapons with [CritH] or [Acc] bonuses - if you go over the maximum chance to hit, extra accuracy becomes increased crit chance and severity).