If you are in the US you might try Lowes or even Amazon.com. I'd found a website in the US earlier that had reviews on shower/tub surrounds but my computer died and I lost the links (I'm on a different computer now). If I find it again I will post the links.
You may want to try doing a search on dealtime and/or pricegrabber because both sites will list products (you can search for "tub surround" or something like that) and have pricing and reviews-- as well as reviews of the stores selling the products.
I'm still searching for something to use when we replace the one piece shower/tub unit that has cracked on the bottom (the doorway is very narrow and will not fit another unit in so separate pieces will have to be used). It's not top priority though, as we have many other projects on the list.
If you find something good, please let me know.