Here's a general list (in no particular order) of various assets and asset stores I browse to find resources for my world projects:

Note: I use the phrase "Free" with quotes to mark assets that are monetarily free but may have contractual strings attached. Use your best judgement when deciding whether to use one of these.

• 3DAsset.One ("Free") - A general purpose "free" 3D asset search engine. Really good if you're searching for one specific thing. Be careful; it includes custom license and sign-up required assets.
• AmbientCG (Free) - CC0 photorealistic materials. My go-to library for any materials I need for world geometry.
• Bakery GPU Lightmapper (Paid) - A paid GPU Ray-tracing lightmapper, I find it generally has a more realistically accurate result than the built-in GPU lightmapper. It has a bit of a learning curve, but I personally prefer it. It's available from the Asset Store for a one time fee, and goes on sale often. It also doesn't have the minimum 4gb of VRAM requirement that the built-in GPU lightmapper has, so if you're building on an older machine with low VRAM then it might be your only option.
• Beautiful Sky by RED_SIM (Paid) - An absolutely gorgeous realtime sky shader by RED_SIM. Just look at this one, trust me.
• Pixiv/Booth Asset Worlds (Free & Paid) - Booth has been one of the best online storefronts for VRChat assets out there. For world creators, they've even made visitable VRChat worlds that serve as showrooms for assets you can download for free or purchase. Although it's mostly paid assets, there are some free stuff floating around the worlds, so go explore. I felt like a kid in a candy store when I first found out about these worlds.
• Poiyomi (Free & Paid) - Great general purpose special effect shader. Often used for avatars but includes a "world" option better suited for worlds. Lets you openly configure and experiment in realtime, then compiles down to a high performant version when VRChat goes to export. Free for everyone, but also offers a paid subscription with early access for supporters.
• Poly Haven (Free) - CC0 photoscanned objects, HDRIs, models, and materials. Most of the models are made for renders, so will need reduction and cleanup for use in VRChat.
• Turbo Squid (Paid) - Most of these models are made for renders, so pretty much all of them will require some cleanup. Some of the models I've purchased have been spectacularly out of date and exported for software I don't have. However their customer services have been spectacular. They offer a free conversion service so if your purchase isn't compatible with your modeling program they will convert it for you in a reasonable number of business days. On one occasion they even offered me a refund on a digital download.
• Unity Asset Store (Free & Paid) - Pretty obvious, but the list wouldn't be complete without it.
• VR.Space Furniture Pack (Free & Paid) - A free furniture set with models built for use in VRChat and VR Applications. There is also a paid version, expanded version of the pack (which I can't seem to find). Most of the furniture in my worlds winds up being from these packs.