Years ago before I started building my RV-6, first flight September 1997, I purchased two 'grab bags' of aluminum sheet from Aircraft Spruce. Was able to get most of the little pieces that I wanted to make without buying much material to make little brackets and parts.
Looking on their website now, they have steel and aluminum bargain bags
but not finding one for aluminum sheet.
I find it nice to have a little bit of 0.020, 0.025, 0.032, 0.040, 0.063, and 0.125 around. Used a piece of 0.125 last week to make a new cover plate to accept a better air inlet filter for my shop air compressor.