Where to get a new or rebuild A/C switch?

We don't know the year and model of your car.

The heater core should be easy. I bought one $25 from rockauto ~1 yr ago for my 65 Dart.

5-button climate switches go for ~$100. A link to one of many currently on ebay:
http://www.ebay.com/itm/NOS-MOPAR-7...Parts_Accessories&vxp=mtr&hash=item1c2bd6e9ab

I bid on one recently at $22, but reserve was higher. Paddock used to sell them for $25 years ago. I couldn't find a 4-button one for my 65 C-body so had to adapt a 5-button. I have stash to last me forever.

Once you find a replacement, I suggest your blower "high" position to turn on a relay, as I did. If you run all that current thru the switches you will melt both the 5-button switch and your L-M-H switch.