looking a kit of stamps for items like fan, rearend, trans, heater box and so on. Where’s the best place to buy them?

In the past I’ve made my own printing them off on paper and then cutting out making a stencil.

There's a guy in Canada who sold those, but I don't have his name/ contact.
If you want to have your own made, the artwork for many or all the stamps you need are available here:
https://www.e-bodies.org/resources/mopar-stamps-stencils-paint-ink-part-numbers/

The bigger question is, what stamps do you need? They were not generic one size fits all, most every car was unique, ink colors, date codes, gear ratios, some obscure unknowns, etc. Your own cars original stampings should be duplicated before removal so that they can be replicated. If you don't have them and aren't sure of what belongs there I'd recommend not adding any back, there are already too many clowned up ink stamped cars out there with imaginary markings. .02

... https://sylvainmopar.com/71sg/stamps.html

That’s real funny. Do you see the stamp for the four speed trans? They used a picture of my transmission.....that was taken in my garage. I did not use there stamp to make those numbers. I made a template and cut it out and used paint. You would think if somebody was going to sell a product they would used that product in there own add. That’s funny. 🙄

Very interesting topic.