Where can one get psychrophilic microbes for a cold-weather digester?

Psychrophilic  or 'cold loving' microbes don't really love the cold, they just do much much better at low temperatures than "mesophilic" (medium temperatue loving) and certainly than "thermophilic" (heat loving) microbes.  They operate from as low as 0 C (some report as low as -4 C!)  to 25 C. They actually prefer 25 C (room temperature) to the colder temperatures so I would say we should re-brand them "ambiophilic" or "Zimmerphilic" microbes.  The point is that they have a lower temperature range at which they can metabolize, due to some glycol like or anti-freeze like properties in their membranes.  That said, they don't produce a huge amount of gas below 10 C and as with most methanogens, the curve is nonlinear meaning you get substantially more gas at 25 C than at 10 C and lots lots more than at 5 C or of course 0 C.  That's just the physics of it.  Everything slows down in the cold.

That said, given that most unmixed biogas systems (and that includes most of the small scale systems from 1 to 10 m3) have a thermocline in them with the coldest area being the bottom of the tank and the warmest up at the top. That is how water stratifies when left to settle (think of a pond or a pool). The top of the tank can be a toasty 40 C and the bottom can be as cold as 10 C and you could put your hand in and feel the point at which it goes from burning you to chilling you.  It is abrupt, with rarely any gradation.

Because of this, we recommend putting BOTH psychrophiles and mesophiles in your digester.

The psychrophiles should establish themselves in the colder lower regions and the mesophiles in the warmer upper regions (we also recommend putting in a lot of vertical surface area, like what we call "microbe motels" or "microbial fuel rods" so they can seek out their own best temperature regime.)

You can try innoculating with thermophiles too, but they are finicky and don't handle temperature changes well.  They way survive in areas where it constantly stays over 40 C, and they give LOTS of gas, but you will have a hard time taming them we think.  The mesophiles are the most tolerant to temperature changes.

But since the bottoms of tanks do usually stay at or well below 25 C, it is good to have the psychrophiles in there, chumming along.  They may not make a LOT of gas, but they will constantly be doing it so they are kind of an insurance policy, and they seem to play well with the mesophiles (maybe they even engage in gene transfer, who knows?).

Where do I get my psychrophiles?

You can get psychrophiles from pond muck, usually from duck ponds (I call it "duck muck") or lake sediments, anywhere where the water has stayed under 25 C all year (as most freshwater water bodies do) and where animals, particularly migrating birds, have sat and shat.  I got some from Lake Eyak in Alaska (in Cordova) and from the thermokarst lakes north of Fairbanks. I also brought some back from a little pond on the trail by Mount Everest Base Camp.  These critters were very well adapted to cold weather, needless to say. But still they didn't produce noticibly more gas than the ones I got from our local Ententeich (duck pond) in Essen, Germany.  Or from the duck pond at 145 Palisades near Curious on Hudson Bookstore in Dobbs Ferry NY.  Some speculate that these extremophiles are all the same, spread on the feet of migatory birds all over the planet. 

The point is that you don't need to go very far to get methane producing psychrophiles.  Katey Walter Anthony from U Fairbanks got some from the hippo wallows in the Okavanga Delta when she visited the biogas systems we built there.  Anywhere you stick a stick in the mud and bubbles come up you can be pretty sure you have psychrophiles.

It isn't a bad idea to have active pond or lake muck in your digester regardless.  Put them in any time and let them settle to the bottom.  Also add mesophiles from any manure source (even human) any time.  The key to a healthy digester is biodiversity.  Keep adding sources of methanogens of any philia, and let them sort it out. That is our advice.