We have a Transtar Transmission warehouse close to where I live in Denver, so it's pretty good for standard rebuild kits and converters.
The standard tan clutches in the 727 kit worked fine in my 12 second Charger, But I had a Hughes converter.
My 360 powered truck has a converter from Transtar, but it is not listed in the catalog. It is a factory highstall converter but had upgraded internals. I think the upgrades are maybe $100-$150 on top of the price of the standard rebuilt converter, which was only like $100 to start with.
They have 6 locations in Texas.
https://www.transtar1.com/

For performance parts, I have bought a few kits off E-bay, but the part types and prices are all over the place so you sort of have to know what your looking for.

I have got parts from some of the other on-line transmission warehouses too,

If you need several different parts, check the shipping costs. It might be less expensive to get all the parts from one source even if the individual costs are slightly higher?