Londero has a brand spanking new safari sidelock Heym with factory mounted Zeiss (I believe) scope in .375H&H. It is a really good value for what it is but it's $25k CDN ( price would normally be around $25k US without mounts or scope according to the posted price list on the Heym USA web site) and it's not in your target caliber. I believe Londero is an importer for Heym so they may be worth speaking to if you want to order up a new PH boxlock in .470.

From time to time over the last 4 or 5 years I've seen a few used Heym box locks pass through Epps in both .470 and .500 for $14k or maybe a little less depending on condition, but they don't stay on the shelf very long.

Best of luck in your search. It may take a while but it will be loads of fun and most certainly worth the effort when you get into your double!!!!!!