I need to purchase a carb rebuild kit for a SB and BB 4-barrel Carter AVS carbs. Any recommendation on brand/where to purchase?

Try allcarbs.com, AKA Allstate Carburetor. The Edelbrock kit for their Thunder series should also work, I think. That's essentially an updated AVS.
Used an Edelbrock kit for their AFB-style carbs on an AFB of mine.

My carb man, John at "the carb shop" (50+yrs exp) said to avoid Tomco kits.

Echlin kits from Napa are great for carters

the two real issues are the top gasket and accelerator pump ('69 and later are larger than '68). I haven't found a good complete kit. some edelbrock stuff will work but pretty sure the top gasket won't.

Kits and kit components are constantly changing, most changes are not for the better. I often end up using parts from various sources to cherry pick the best of each. In general I agree that the NAPA/Echlin kits are good, and also kits sourced from Walker Products in California.

So whats wrong with tomco kits? I have one for my 4640 Carter that I've been hanging on to for years now in case I ever needed a rebuild. It must have 50 or 60 little tiny parts in the still sealed bag.

I don't have the answer for that question as far as actual experience as this is second hand info from him so I put it out there like that as third party info (but I was very impressed with him) as I was way too busy trying to write down/remember all the excellent tech he was laying on me (for free) to ask specifically but I would assume poor quality/fitment. I'm sure he will tell you & he takes calls on Monday & Tuesday only. John 573 392-7378 I'm assuming inferior quality of some sort & I have seen many gaskets in various brand kits that the holes are slightly off especially cluster gaskets. No overseas brass floats was another caveat he mentioned (poor quality solder that can leak over time iirc).

The last oem Carter carb I rebuilt I used the kit from mikes carburetors. Worked perfect. Customer likes it better than his fuel injection conversion.

I've seen some crummy components in Tomco kits. I've always had good luck with Standard/Hygrade/Sorensen rebuild kits - all 3 are from the same manufacturer. I'd use them on anything, except 4150/4160 Holleys - there are better makers for a holley, including holley. The last AVS kits I got some years ago came from Year One and they were OE Carter kits.

Tomco carbs and kits got junky in the early 90s while I was in a USAF motor pool buying repair parts. Anytime carb work was required, a Tomco rebuilt and an Echlin kit would get requisitioned, then the rebuild was rebuilt and installed and tuned. Heck Tomco couln't even get the floats set right.

Pretty bad when voiding the warranty was a non issue.

(Also I learned you can overheat a 727s and TH400s enough to melt the case, but that is a story for another time.)

Here is where I have gotten all the parts to rebuild my AVS carbs. They have all the "kit" parts and stuff that doesn't come in the kits.

I don't think the gasket for the top/bowl is the same , looking at ta thunder series it's a performer with the rear butterfly weight plate removed and a spring loaded air door added , it still has the AFB style rear venturi setup and a real AVS does not.

the edelbrock avs is modeled after the the mid '60's chevy avs. the only major deviation from the chevy avs is the fuel inlet. a mopar avs top won't fit on any edelbrock carb without mods. I wish there was a way to buy the edelbrock avs top because I want to convert a 1407 edelbrock to a mopar configuration.

+1 for QuadrajetParts. I used them for both my AVS and my Caddy Q-jet, they are a quality operation.