I think you should start looking at your hometown antique shop first and start from there. or if you can, you could create you own salt and pepper shakers and make them look antique!
One good place to begin collecting salt and pepper shakers is to check out thrift stores. You never know what will show up. Another good place would be a flea market or garage sales. These are places to find things cheap and to make sure you like the hobby.
My grandmother started her collection by visiting garage and yard sales. She would find all kinds of shapes and sizes. She increased her collection by visiting the local flea market.
Other peoples junk is other peoples treasure, so start with yard sales and the local thrift store. These items are priced cheap, just make sure they're unique and in good condition. Google them, see if you have a match and if you don't donate them back to the store. You just may luck up and get that treasure.
There are several ways in which one can start collecting antique salt and pepper shakers. This includes scouting out garage sales in your city for people selling these items. Also, antique stores in your city are a good idea. Also, try looking online on sites such as Ebay for them.