Drop, keychron, epomaker, kono, kbdfans, kprepublic, keebd, keebio, splitkb, aliexpress, ergomech, and many many many more

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.

Give us your country, then people know what to suggest.

I always reference the vendors list from keyboard builder's digest.

Other people have already answered your question, but Amazon has a shit selection for keyboards.

Got mine from Keychron and I absolutely love it

My Zoom75 is coming from keebsforall. Should be here Tuesday (finally!)

Rndkbd for switches and lube especially

I like Flashquark, decent prices and good support.

Taobao, Lazada, Shopee.

What, don't have access? Then as someone mentioned, list where in the world you are from.

PS. Taobao is seriously good if you have access though.

NuPhy make the best pre-builts, head and shoulders above Keychron in my opinion.

You can order directly from them. Don't buy keyboards from Amazon.

mech.land and geon.works

most brands have their own shop. shipping will take longer tho, because they often ship directly from china.

Can't speak for mechanical keyboards specifically, but finding what you want for the cheapest price is most often done best on eBay rather than Amazon.