Do recognize that putting line voltage into an environment that is wet
(your's may be very dry, I don' t know) is risky...

A simple heater might be easily made from a wall wart that gives 12 to
14 volts AC at about 1.2 (or more) amps. Use a 15 watt resistor (maybe
Radio Shack) that is about 10 to 14 ohms in resistance. An AC wall
wart will work well in this application. That will limit voltages in
the terranium to a safe value (don't want your livestock chewing on
the cables, right?) An improvement woudl be to clamp the resistor to
an aluminum plate (say 4 x 4 inches or 100 mm square) that is perhaps
an 1/8" thick. That will help spread the heat out and limit hot
spotting some.
======================================
You might consider using heating tape which produces heat in the order
of 15 - 20 Watts per metre . It is used to keep water pipes frost free.
For a short length as probably required for your terrarium you need a
transformer with voltage outputs in the 5 -24 V range , with a number of
voltage taps. This heating tape is insulated and can be found at
industrial electrical components suppliers.
For safety reasons do not apply voltages above 24 V and make sure that
metal parts of the terrarium are always earthed whatever the heater
voltage.

Frank