Prebuilts from brands like Ducky? mechanicalkeyboards.com

DIY kits are generally best bought from their mfg’s - Akko, Keychron, GMMK, Keycult.

More general retailers will have some mix of keycaps, switches, kits, maybe prebuilts and accessories - Novelkeys, Canon Keys, Mekibo, Bolsa Supply (mostly GBs), Vala Supply, KBDFans, KPRepublic (mostly caps), Kono Store, Drop (mostly caps).

To track upcoming and live GBs, at least for keycaps and keyboards - mechgroupbuys.com. It’s pretty dead at the moment but I think the number of active group buys has tumbled recently due to several high profile scandals in the industry (namely Mechs & Co and Rama). Also several major keycap manufacturers piled up huge backlogs during the pandemic that are only just finally clearing up, namely GMK and Milkyway. All of that is causing there to be very few new GBs starting right now, and they can be kind of hard to find unless you’re already in the communities of the designer(s) or refreshing Geekhack.