Can anyone point me in the right direction to locate a rebuildable non A/C heater box for a 1969 Dart?

Parts wanted or parts for sale on here.
Ebay.
Ad in your local Craigslist.

They are fragile so if you buy one and have it mailed to you the stars have to align for it to show up undamaged.
I bought two online and ended up with broken ones so I gave up trying to get one that way and was able to get one from a local mopar guy who was parting some cars out.

I agree. NEVER have one shipped. Pick up at a show/swap. Ask how I know.

And if you DO have it shipped, have the heater core and blower motor removed and shipped separately. The mass of those 2 items will crush a nice box even if its packed in 3ft of bubble wrap.

Lastly, again if you do decide to ship, DONT USE UPS!!!!!!!!!!!!!!!!!!!!!!!!!!!!! UPS is my primary shipper but those guys, that get paid FIFTY DOLLARS AN HOUR now with the new contract, are SO ROUGH on packages, that there are some items I will not ship with them. My former UPS guy would walk across the top of a pile of packages.

I have a set of E body cat whiskers right now that I filed a clam with UPS. We put in a cardbord tube and then inside a box. They were bent in half. They make you take a dozen pictures, in a specific sequence (or else you lose), ask questions on the PHINE BOOK SIZED form that dont apply to every scenerio, and then I have to show my invoice from where I bought them and thats what they reimburse. Not what was paid. They self insure so they have NO desire to pay out a claim. What a [censored] outfit but thats our world now.