There are quite a few products out there that can help produce the look you are going for. As mentioned,
Woodland Scenics has as full range of ground cover that can be used to make some awesome bases.
Necrotales has some good tutorials on swamps and the like. The page hasn't been updated in years, but all the information is good.
Secret Weapon Miniatures offers a swamp/wetlands basing kit that can produce some sharp looking bases. Misterjustin (the owner) even has an awesome tutorial video on the page. Definitely worth a view.
Hope this helps, good luck!