Palette cups: there are several major problems with the palette cups generally sold by art suppliers. First, they're always teeny, secondly, they get so gunked up with medium so quickly that it's hardly worth spending money on them and thirdly the clip on the bottom is always too small to clip to anything other than the flimsiest of palettes. Solution: find a catering wholesaler online. Buy a small steel sugar bowl (the kind you get in cheap cafes - get half a dozen if you want to make a few) and some sprung table cloth clips (this should come in at less than £5). Glue a clip to the underside of the bowl using a strong super glue and voila!
Mahl sticks: there are some very expensive and fancy versions available online but a 3 ft piece of half-inch dowel works just as well. If you're worried about damaging your painting surface you can pad the end by getting some fabric (gauze bandage works well) - taping and/or stapling the end of it about an inch below the end of the dowel. Wind the bandage repeatedly round and over the end of the dowel to make a padded end. Secure with tape and/or staples. Cover the entire thing with a circle of chamois, held tightly in place with a thin cable-tie fastened around the dowel where the padding stops.
Taboret: the top-end art suppliers charge a fortune for taborets (a.k.a. artist's trollies). I've been tempted in the past with getting one of them but places such as Ikea, Robert Dyas and the Range do kitchen trollies at a fraction of the price which work just as well.
Black mirror: lots of ateliers recommend students use a black mirror to judge shapes and values. The black mirror is a derivitive of the 'Claude Glass', a small mirror, slightly convex in shape, with its surface tinted a dark colour. They were used by artists, travellers and connoisseurs of landscapes and had the effect of reducing the colour and tonal range of scenes and giving them a painterly quality. The trouble is it's difficult to find them for sale. A simple solution is to get a cheap A6 size clip-frame and a can of matte black spray paint. Remove the glass from the clip-frame and spray one side of it. Allow to dry. Replace the glass, paint side down and clip in place against the back-board to protect it from bumps and drops.
Sanding blocks: the sanding blocks sold by art suppliers are usually expensive and really poorly made (just a few bits of cheap sandpaper stapled to a strip of board. As an alternative, you can get foot files from online health stores for a fraction of the price which are much stronger, last far longer and normally have two grades of surface, which can come in handy.
Picture frames: not such a new idea - artists have been doing this for years, but worth sharing anyway for those who have not cottoned on. By far the best way of protecting your pictures is to get them framed. However, if you have studies and drawings you want to protect which don't justify bespoke framing, you can pick up good quality frames in charity shops. Most people look at the picture inside the frame when they price up these articles. Since the pictures inside are normally hideous (!) you can get some absolute bargains if it's just the frame you're interested in. Remove the offending picture (usually this will be of a log cabin by a waterfall, a child hugging a dog or a WWII fighter plane), clean the glass, dust the frame, cut a new mount if necessary, and fix the picture inside with the original board or a new one - held in place with glazier points (available - with the tool to push them in - from most hardware stores).
Well, that's my selection so far. If I come across (or invent) any others I may do a follow-up post on here. If you have any art hacks you'd like to share then do let me know!