To be honest, some of my best plant finds were in a PetsMart. The key is to know the species that you are looking for. You can't just walk into those types of stores and randomnly select plants that please you, and you certainly cannot rely on the workers to help you. They are clueless. You need to research what is appropriate for you tank and what is truely aquatic. My lobelia cardinalis is from PetSupermarket, another chain store, as are my Amazon swords. Here is a list of the plants that I have obtained from Petsmart.
Lobelia cardinalis
Cryptocoryne wendtii
C. lutea
C. lucens
Marsilea hirsuta
Hemianthus micranthemoides
Alternantera reineckii
Bacopa caroliniana
Egeria densa
Cambomba species
Java fern
Water sprite (Ceratopteris thalictroides)
and the kicker...
Hemianthus callictrichoides at $4 a pot. Now, that isn't cheaper than online prices, but I didn't have to pay for overnight shipping, which would make it about the same.
So, you can find a lot of things there, if you know what you're looking for. All of the above are aquatic plants.
I will say, however, that in a new Petsmart, I saw a disturbing trend, they no longer have the aquarium that houses the plants. They now sell mostly non-aquatics in little tubes. This is disappointing as they are blatantly calling them aquarium plants. I spoke to the manager about this when the store first opened, but I was ignored. I only hope that the older Petsmarts do not follow suit. At the hands of a well-maintained store, the true aquatics can be a bargain.
The non-aquatic plants always gets to me, if you want a good time TRY to explain to them that they're non-aquatic..
Oh, I just show them pictures of my houseplants on my cell. When they see that they are the same species, they usually shut up pretty quick. In fact, most of them are outdoor plants in my neck of the woods. Still does't stop them from selling them.