I found a garter snake, checkered I think, up in the mountains (about
9000ft) in south-eastern Arizona. The hotest it ever gets up there in
the pine trees is about 80, and usually is less than 70, and of course
it even snows in the winter. My house is kept between 65 and 80,
depending on how well the swamp cooler is working (yuck!). Shouldn't
this be fine for this snake, without any extra heat? He is in the
living room, so has light during the day, and it is dark at night (no
special or bright light, just the daylight and regular room lighting).

I've put his cage on top of my king snake's cage. I've had the king
for 7 years, and he has done very well under these conditions. Other
snakes that I've had in the past, with hot-rocks and/or light bulbs,
haven't done very well. However, that could also be the difference
between store bought vs wild. The native/wild snakes always live a
long time, but I've never had one from the pet store live over a year.
The only ones I've ever bought from a pet store have been ribbon
snakes, boas, or pythons. I imagine boas and pythons need more special
equipment to reproduce their native habitat.

The guy at the pet store said it is fine to feed the garter snake just
guppies, but I've read several posts in this group saying they also
need worms or something else and can't survive on guppies/fish alone.
Where can I buy worms that are small enough for this baby to eat, he is
only 5 or 6 inches long, and even the tiny guppies are as big around as
he is (almost). He did eat up about 5 or 6 guppies within a few
minutes today, which I was happy about. I know Walmart sales "White
Mountain" NightCrawlers, which I don't think are toxic, but they are
pretty big. I think they may also sale smaller earth-worms, also for
fishing, but I haven't looked at those yet (I always fish with
crawlers). How about tubifex (??) worms, the fish food kind, would
those be good for him?