Copper sheet should be part of every DIY'rs parts store:
ground plane - make flawless signal return ground path using bug-splat! type connection points.
shielding/lining chassis, sensitive high gain stages, transformers etc.
And make your own tube shields - commercial tube shields are perfectly useless thin .3mm aluminum. Roll a tube shield from this copper sheet, run a ground wire to chassis and you won't believe the difference electrically and sonically.

This sheeting is 0.4+mm thick, the thickest that can be reasonably soldered by DIY'rs because copper is VERY effective conducting heat when in sheet form.

The tricky part is how we sell it, which is by units of 28cm2 (100g):
Minimum quantity is 28cm2 (ie 4 x 7cm or any other dimension that multiplies out to 28) and sells for $5USD
We will cut it with unfinished edges to any size up the the sheet width: 305mm

So first decide the size you want. ie 5cm x 28cm (good for about 3pcs) 12AU7 tube shields
Multiply the area (5 x 28 = 140cm2) and then divide by 28 = # units to order

example:
A sheet of 5cm x 28cm = 140cm2.
140/28 = 5 units
so you would need to order 5 units @ $5USD and then include a note in your order specifying the size needed ie 5cm x 28cm

We will cut it with unfinished edges to any size up the the sheet width: 305mm