RAST bedside table from IKEA - £9.00
Pair of 8U or 6U rack-strips - £7.00
(Depending on how many units you need). I bought this from eBay, but you may be able to find it in a music shop. Alternatively you could just saw up two lengths of wood and screw the instruments into these using wood screws.
Rack nuts and bolts - £3.00
Again, I bought these on eBay, but you'll almost certainly be able to buy them at the same place you buy the rack-strips.
Various wood screws - ?
You'll need a few short wood screws to secure the rack strips to the table. I also needed four long wood screws to secure the newly-lowered shelf (not necessary if you don't lower the shelf!) I already had all of these screws.
Furniture castors - £5.00
I already had these left over from a desk I was throwing out, but you'll be able to get them at a hardware store.
Drill and drill bits