The best thing to do is pull a new thermostat wire. Home Centers sell rolls of thermostat wiring and it's fairly simple for a DIYer to install it. Count the wires being used by your current system and buy a roll that has at least one more wire than that. So if you have 4 wires now, buy a 5 or 6 wire roll.
If that doesn't seem feasible, you'll need to buy a Nest Power Adapter. They sell those wherever Nests are sold.
https://store.google.com/product/nest_power_connector?hl=en-US
