just a quick list of folks I get my most commonly-used materials from; click the banners to find their websites...
I've sculpted with milliput ever since when. Probably my favourite medium.
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epoxy putties galore - this is where I get my greenstuff for miniature-work
also greenstuff, plus many great sculpting tools and resources
sundry supplies for casting, moulding, modelling etc.
resins, rubbers, and various other moulding and casting supplies
composites and other materials for modelmaking and prototyping
kits and accessories for every kind of crafting you can think of
another avenue to explore for your crafting requirements