Could someone please tell me how to locate three GE 6CG7 tubes for my stereo? I am new at this. Sincerely Vanda

Hello, I am new at this and don't know how to locate parts. So far, I need three tubes-6CG7. I don't know a lot about the stereo that I have. I have a receipt which states that it was bought brand new in l962. I am anxious to get some help with this. Sincerely, Vanda