Cheap Undies
Cheap Undies is a great site if you want decent underwear in all styles, including jockstraps! And since they carry all sorts of brands, you're guaranteed to find something above a 36 inch waist. Their stock comes from all sorts of designers, from known fashion brands to unknown brands making their mark. Worth mentioning is that they also carry sexy underwear for you to show off to your partner, or just for you and your selfies. The unique thing about Cheap Undies is that they have daily discounts on something new each day, so it's worth it to put it in your regular website rotation.
Fresh Pair is a premier underwear shopping site, carrying everything from affordable brands likes Hanes to high fashion brands like Diesel. For big guys, they have a big and tall section, and the search sidebar makes it easy to browse. The sheer variety that this site offers puts this site above all it's peers, offering all sorts of styles and they also including other innerwear such as socks and t-shirts. They offer free shipping for orders $25 and more and regular run sales.
Amazon
Amazon is also a great choice to shop for big guy underwear, considering they sell everything else. They sell a variety of brands in all sizes and what's great is that they also have a dedicated big and tell section to make your life easier when you search. With Amazon Prime, it's better if you choose Prime products and get the faster shipping and possibly a discount.
Department Stores
You know department stores? Those stores that anchor your local dying mall? Check out the men's underwear sections, they usually have great finds in there! Department stores usually carry high-end premium brands, so while they may be pricey, they're made of higher quality material. These stores also have dedicated big and tall sections for your browsing.
Hey, sometimes you don't need fashionable or premium fabrics. Maybe you just need a few 3-packs just to get you through the week. Stores like Wal-Mart and Target serve a purpose and that's providing affordable underwear available for all sizes. Also, Wal-Mart and Target have exclusive brands (Gildan and Pair of Thieves respectively) that are worth checking out. They don't usually have big and tall sections, but for waistlines 40-50 inches, you should be able to find a pack to your liking.