There's a very large selection of terminals of that style. Here are ones that look similar:
http://www.digikey.com/product-detail/en/1776244-3/A97997-ND/1826860For larger gauge wire, try something like these:
http://www.digikey.com/product-detail/en/796740-3/A98276-ND/1827129If you're still looking for information regarding the relays... the voltage needed to switch the relay (the control side) is called the "coil voltage", and you can search relays on digikey by this property. The electricity you run through the relay (not the coil) can be at any voltage/current, AC or DC, within the specified ratings of the relay.
The relay linked by Xenland is a nominally 5V rated relay. If you look at its datasheet, its coil can be controlled by voltages as low as 3.5V. If you need voltages lower than that, the lowest voltage power relays on digikey currently in stock appear to be these:
http://www.digikey.com/product-detail/en/TX2-LT-1.5V-TH/TX2-LT-1.5V-TH-ND/2709469You can drive their coils with voltages as low as 1.125V.
Another important property of relays is whether they are form A, form B, or form C. Form A relays are normally open, Form B relays are normally closed, and Form C relays have 3 pins: a common, a normally open, and a normally closed pin. To understand the terminology, normally open means that the relay does not conduct when the coil is unpowered, and conducts when the coil is powered. Normally closed means the relay conducts when the coil is unpowered, and does not conduct when the coil is powered. Make sure you know which you need for your application and how to wire it up correctly.
Let me know if you need any assistance with your design.