Source Needed: Soft Aluminum

The bars sold in the metal displays in the better hardware stores are about the softest I've seen. Easily bendable, but they do have some temper. I used to use 2024T4 for pack stays, only because it was stiffer than the hardware stuff and held its shape better to transfer weight, but still could be bent to desired shape.
Your mention of hobby stores is probably right – the aluminum stock they sell tends to be quite soft for making models. I've only seen much smaller dimensions, though (for models).
Good luck.

Don't know if this is exactly what you want, but I just looked at my finger that is wearing an aluminum splint. It is extremely soft, bendable, and about the dimensions you describe. It has a layer of foam that would have to be removed, though.

Yep. That's the stuff.

I just don't know where to find it in the required dimensions.

Google Prosthetics and Orthopedic for the largest town/city near you. Go visit them and ask for what you need.

I googled it for my town and came up with 6 places that make custom items.

Since you're still looking, try 'busbar' used by electricians. It must be soft to have the high conductivity. Maybe it will be a little bigger than the jewelry and orthopedic stuff.