Susana at Montessori Candy asked me about the materials we're using in the classroom. I got them from Montessori Outlet, and so far I'm really pleased with everything.
My daughter's old school had all Nienhuis materials, and while Montessori Outlet doesn't carry "official" Nienhuis stuff, on their website they claim that they carry "premium-quality materials at outlet prices," and that "all of our products are made based on the authentic blueprints approved by Mr. Bert Nienhuis. All of our new premium materials are 100% compatible with Nienhuis materials."
Also, all of the materials are lead-free and non-toxic.
So far, I would say I'm totally satisfied with everything. I did a lot of research before choosing a supplier, and (at least for a package deal) their prices were the lowest to be found (I can't vouch for "a la carte" purchases). The materials are beautiful and feel very substantial in your hand, and they have an inherent appeal that makes them hard to resist - you just want to handle them.