Real 6.5X53R dies are rarely found used. The best prices for new dies are probably from CH4D.
The 6.5X54 Mannlicher Schonauer is nearly an identical die set.
The back end of the 6.5X53R FL die is about .005 larger.
If you have a way to spin the die you can polish the back half of the die out until it fits .303 Brit brass.
To make the brass you start with new RP or WW brass in .303. These cases are .450 to .452 just ahead of the rim. PPU brass does not work because the head is too large at .454 to .455.
Round up the case mouths with a .311 expander.
Then push the shoulder back with a series of .308 Win and 7-08 and 6.5 FL or trim dies. Then FL size and trim to length.
PM me for more detailed instructions.
P