I was thinking an OG base set, I had no idea so many options were available…
If you want something that looks almost identical to the base set charizard you could consider getting the 2021 celebrations charizard which is a reprint of the OG charizard (even same set number etc.) and has the 25th anniversary stamp on it. This is a lot more affordable and only costs around ~250$ in a psa10
There is also evolutions charizard which has the same art, but different attacks etc.
And clc classic collection charizard which again is like the base set charizard in terms of stats, but different set number
Another very cool option is the metal charizard card from the celebrations upc. This looks exactly like the OG baseset charizard (even without the 25th stamp), but is made out of metal and is "golden". It's not a normal pokemon card though. This is one will be very hard to get as psa10 though if that's something you / your son cares about. So you might have to settle for a 8 or 9
In terms of buying I would say eBay is generally a good place, at least in europe i've made good experiences with it. I'm not sure what the most common peer to peer card selling platform in the US is, i heard tcgplayer mentioned in many youtube videos, so this might be it.