I've replaced mine at least once.  You can find the complete
replacement kit, including battery, o-ring, and cover for US $10.99 +
shipping at:

http://www.suuntowatches.com/Suunto-D3Mosquito-Replacement-Battery-Kit-MOS.pro

 I've changed my Vyper battery without changing the o-ring, more than once.

 I dunno if that makes it smart,....

I would like people who own Suunto computers to be aware of the
following:

I don't trust Suunto on their computers.  First they have known
problems:  http://www.suuntoservice.com/safetynotice/diving/
and second they have known problems they don't speak of such as a
freezing problem of their Cobra computers.   I know because mine, S/N
527334 froze at 85ft on me.   no buttons worked and even when I
removed it from the regulator, it still showed the pressure of over
2500PSI.   I returned it to my dive shop who informed me they had at
least one more returned for the same reason.   It's been at the shop
months now and I finally found out because I didn't provide my sales
receipt, they didn't do a thing.  Despite the fact that by the S/N
they know it's less than a year old plus the unsafe condition of the
device, you'd think they would have done something.   Additionally
there is nothing on their website about this problem.   Of course no
one asked me about my safety either so apparently Suunto isn't
concerned with your safety or the safety of their dive computers nor
are they willing to share their safety issues online.  I would never
even consider buying another Suunto dive computer.   Suunto used to
make great oil filled compasses but my suggestion is not to trust them
with your safety using their dive computers.    I have two Oceanic
dive computers also, one VEO 250 and one PRO and have never had a
problem with them.
--Norman