You can get mini mist-tanks (restore 15 ME) by by referring steam friends to spiral knights (and a costume hat after 5 referrals).
As of the release of King Krogmo's Colliseum, it is now possible to also get mini-mist tanks as a Krogmo bribe reward, a process which takes place before each PvP match. The odds are rather low, however.
You get a single mist tank when you first arrive in Haven (and cross the "Final Mission" from the starting area) and (effectively) earn Tier 1 Clearance. Likewise, you will also get a mist tank for achieving Level 2 Clearance, and for achieving Level 3 Clearance as well.
All three of those instances are one-time endeavors.
Finally, if you buy the Starter Pack for 20$, you will gain 5 mist tanks. You may only buy one Starter Pack per account. This means the maximum number of mist tanks obtainable is 8 per account.