Thanks for all the tips and the links.
I hadn't considered making my own marzipan since making/assembling the petits fours themselves seems to be quite a process. The recipe I found suggested just buying marzipan and fondant.
But the recipe you linked to seems easy enough, so if I can't find it in one trip to the grocery store, I may consider making it myself. Have you tried making it before?
After doing some internet searching, I'm a bit confused about the phenomenon that is fondant. It seems that there are some variations of it (rolled vs. poured), and the stuff professionals use has a lot of added stuff (glucose, etc.) that makes it difficult to make at home. I have also seen recipes w/ and w/o powdered sugar added.
I came across the Wilton site in my searches yesterday, but they only sell rolled fondant. I think I'm supposed to used poured for petits fours?
Also read suggestions on one site for covering cakes w/ buttercream before the fondant to make a smoother surface, but I think they may have been referring to wedding cakes.
Basically, I'm trying to recreate to a degree these really good petits fours my mom used to send from Williams-Sonoma for my bday. The icing on those didn't seem like just a glaze, which is what worries me about the homemade recipes. There may have been a buttercream layer, too, but I can't remember.
Needless to say, this is a much more complicated project than initially anticipated! But I think I will try to give it a go and see what happens. Will report back...
In the meantime, if anyone knows of a good bakery I could ask a few questions of, please let me know!