What is the best (and most inexpensive) indoor latex primer to use? One entire room is new drywalls & ceilings, the other is existing walls but patched with spackle. I think I need about 4 to 5 gallons.

These terms " best (and most inexpensive)" do not fit together. Do you want the best or the cheapest, opposite ends of the spectrum. Go to a big box store and buy a 5 gallon bucket of contractor grade primer, runs about $45

Where do I find this paint?

"big box store " Lowe's, Home Depot, Menards, etc... Do you have any of these near you, where are you located?

Thank you.