I turned the power off at the breaker and turned back on and now the thermostat and the unit have no power

What caused you to turn off the breaker, were you already experiencing an issue? Use your test leads right at the breaker, if you verify 120 vac then open the furnace panel and confirm 120 is reaching the control panel.

I turned it off because I was expecting to have to take off and clean the heat sensor. Actually, I just put the back panel back on and the power came back on to it and the thermostat. Looks like I have a cracked ignitor. Any suggestions on where I can go to purchase a replacement, vs. ordering online?

Yep, there is a safety switch on that panel and it may not have been fully seated before. New ignitor, what model and brand do you own and where do you live?