For me, it’s the following:
Hobbylink Japan - But they have disappointed me MANY times over the last few years when I pre-ordered items months in advance, and it goes straight to “backordered” as soon as it is released. And then no restock for months, if at all. I mean the good stuff I want, not the junk that is in stock. Plus, shipping from Japan has gotten very expensive. Bandai stuff is getting so hard to buy from them that I have resorted to paying a little more from US sources, like USAGS.
USA Gundam Store (USAGS) - So when HLJ fails to deliver the Bandai kits that I want, I go here.
Premium Bandai USA (aka Pee Bandai) - Bandai seems to have switched their product focus towards items that are only available from their website. I think it’s a dirty money grab scheme, since they are cutting out the retailer. But if I want something they have, I either buy it from them or get scalped by third party sellers. I could write volumes about Bandai’s current evil business practices but will refrain since this thread is about where you can get stuff. And I do admit to buying most of my Gundam kits from them nowadays.
Sprue Bothers - They are pretty good about having stuff in stock, so I am increasingly relying on them for my “normal” i.e. scale kits and accessories.
Hannants - I go here for those hard-to-find European kits and accessories. Plus, with the British Pound being weak against the US Dollar, Airfix kits are a little cheaper than in the US.
Hobby Terra - A Ukrainian outfit that amazingly is still doing business despite what is going on over there. I get most of my Ukrainian kits from them, like Amodel, Modelsvit, etc. But since the war started, the flow of new stocks has slowed considerably.
Ebay - When I’m desperate for that one hard-to-find product that I simply must have.
I also have dealt directly with manufacturers like 26 Decals, Aerocraft Models, Draw Decals, and OKB Grigigorov.