The Works has some good craft stuff, usually quite cheap.
Some suggestions: pipe cleaners (coloured ones are more useful than white), googly eyes, feathers, sticky jewels, sequins, cotton reels, buttons, craft foam, cardboard from packaging, glitter glue pens (avoid loose glitter - it WILL get everywhere!), tissue paper, mini pom-poms. Child-friendly scissors that actually cut.
Would cellophane work for the project your DS wants to do? I think they sell it on Yellow Moon.
Google activity village its fantastic loads of printables and good ideas. Our local £1 shop sells load of stuff for crafting. Have you seen the printable cars you cut out to make a 3d car? Other items Pasta shapes, Glue, Argos cat for pics etc, Bog roll, PVA glue, paint brushes big and small, modelling clay, Dads old shirt for an apron, Grass seed and empty eggs for Eg heads, Sponge cut up to make printing blocks, Spray paint can be good with the correct supervision, stencils.
I've always enjoyed doing crafty stuff with DS, but mostly have bought things in the past for a specific purpose e.g. cards and foam stickers etc to make Christmas cards.
He's recently become obsessed by Art Attack and has a long list of things that he wants to make. I thought I'd treat him with a craft box full of things so that we have the stuff to make some of these creations, so any suggestions of what is essentials are beyond paper and paint? We've got some toddler bits and pieces like finger paints and hundreds of Christmas stickers but that's about it.
Something he wants to make needs tracing paper (so that's on the list) and clear but coloured plastic sheets (but I couldn't find these on Yellow Moon - not sure if they are called something specific).