where do you purchase stamps in the PC Walmart

Yes at register.

I went to Walmart today just to get a couple of few things

At the Service Desk

I am not sure, you could call them to verify they sell stamps.

Customer service Indy get a book of 20 at customer service as well

You can get stamps at any of the cashier registers or at the Service Desk registers. They are only sold by the book (20 stamps).