I use brass shim stock, the last round I managed to work with 0.001". I gather you're using the needle diameter as a size reference, but with dimple and sand, the diameter isn't that critical. The sharp point is used to create a bump, not size the hole. I usually lay the brass over a piece of smooth mat board as a work surface when I push the pin down to create a dimple. I then use wet-or-dry sand paper wet to work on the dimple. Using a sort of random circular motion I generally start with #400, then #600 as I get closer to final dimension. (I've not gotten obsessive enough to go above #600.) The pin/needle can be used to gently raise the edge of the hole a bit to sand more off if the hole is two small. The object is to wind up with a knife edge to the hole. I agree that brass works better than aluminum and two mil brass is easier to handle than 1 mil.

Hobby shops (especially the model RR oriented ones) often have a whole display rack of brass pieces used for model building, one of the standard items is a kit of brass shim, usually about 3x5 inches and in several thicknesses. Some auto parts places, and hardware stores carry shim stock also. At the model RR hobby shops you can find Blacken-It, a mysterious liquid that blackens brass nicely which is a good touch for a pinhole. Unfortunately the minimum size is about a 3 lifetime supply for pinhole work, and costs about $9 IIRC.

And yes, making the cameras is probably more interesting to me than using them! But I do go out every year for WPPD.