Here is a spreadsheet of (somewhat arbitrarily chosen and certainly not endorsed) online North American vendors of rocket kits.
Some of these vendors also are producers of rocket kits, but I haven’t included in the spreadsheet producers who sell only their own kits — all vendors listed here sell kits from several different producers.
The vendors and their web site URLs are shown in the first two columns. The remaining columns cross reference which kit brands are sold by which vendors. Or rather, which kit brands the vendors appeared to be selling the last time I checked. These things change, of course, and I make no guarantee the spreadsheet is fully up to date. In fact I pretty much guarantee it isn’t.
Corrections and additions are welcome, naturally. (Note, though, that I’m not at present considering vendors outside North America — just to keep things manageable.)
Link to the spreadsheet: https://docs.google.com/spreadsheets/d/15thw_W9lN6rYGx5JtJeJlGr0nagMMGbB9860SL-UAyY/edit?usp=sharing