A peac0ck isn't a spearer anyway, its a smasher.
http://www.stomatopod.com/
Look around there. I personally don't think many, if any, mantises would work long term in a 2.5 gallon...
Smallest commonly found mantis I can think of is wennerae, and even they get 3 inches...too big for a 2.5 gallon IMO.
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Current setups: 1800 gallon koi pond, 10 gallon planted, 150 gallon reef, other FOWLR tanks