Looking for some copper base gasgets for my 250 and 500

Who sells them?

You mean that you are looking for a copper spacer to go under the cylinder.   You could make the spacer out of any metal and obtain the same results.  The sealer that you put on both sides of the thin metal gasket (spacer)  is what makes the metal gasket seal or an attempt to seal.  Copper is not soft enough to compress and conform to any microscopic imperfections on the aluminum  gasket surface of the cases and cylinder base unless it has the embossed or pressed ribs like we see on steel head gaskets.

Gaskets should be made of a material that is much softer than the material it is sandwiched between.

If you are blowing base gaskets on the 500 under the reed valve, it is because the cases are typically not flat in that area.  A metal base gasket will not blow out in this area but it make not seal unless you use case bond.  If  you use case bond on the base gasket you may need dynamite to get the cylinder off of the cases next time you do the top end.