Check Amazon first: typically Amazon's low price is the price to beat. Camelcamelcamel.com gives you Amazon's price history for the last 15 months. Run a Google shopping search. Run your own search on eBay. If the G-Shock you want is a Japanese domestic market (JDM) watch, check Rakuten Global Market. Walmart has clearance sales on sub-$100 Casio and Timex watches, but the sales are store managers' options, so check in your area. For out of production G-Shocks, check the Bladeforums Knife Exchange, individuals and dealers, for Gadgets & Gear.
I think that covers all bases.