To search a specific shop on Reverb, you'll need to know part of the Reverb URL for that shop. For instance, the shop URL for Chicago Music Exchange is reverb.com/shop/cme. So to search pedals in that shop, you'd type "pedals shop:cme".
To search for listings in various countries, you need to know the two letter country code, such as CA for Canada. Here is a full list of the 2 letter country codes on Wikipedia.