These are the websites we have been using for years.
www.absorbentminds.co.uk
www.montessoriservices.com
Both these websites offer a wide selection of Montessori materials and useful child-sized materials such as wooden trays, glasses, and aprons.
www.nienhuis.com
Located here in the Netherlands, Nienhuis is an educational supplier of Montessori materials
www.rolf.nl
I like the craft materials available at Rolf’s website. Again an educational supplier, but also allows orders from individuals