Does anyone have some tips for getting larger light fixtures like the ones in the title?

I've always found it really difficult and a little expensive. When I got my troffers (brand new) online, it took a lot of searching and the shipping was a lot. I've also always wanted some louvers, but they seem even harder to find online.

Keep an eye on renovations and demolitions. If you can talk to the site foreman and be ready to offer cash, you might get pleasantly surprised. That is how I got my louvred fixtures.

Troffers are easier, you can probably still find them a lot on sites like FB Marketplace. Renovations and demolitions are a good source too as are change outs.

As for louvers those are harder to find. Sometimes you might also seen them on FB Marketplace as well.

I agree that local demolitions and renovations are a great place to source big fixtures, as well as enquiring about fixtures at in-use buildings - some of my most recent fluorescent fixtures, while not troffers, were sourced from my own school's lighting overhaul completely for free (although I did get thank-you gifts for those who helped me). Especially older commercial or public buildings generally have great stuff hidden out of sight such as in a rarely used basement or back office.

In fact, I recently had time to stop in a local business that had 6 prismatic troffers of unknown lamp type and lamp count (it looked like they were all 3 lamp however I cannot be sure as each troffer only has one lamp working), and let the friendly business owner know that I was interested in having them should he ever replace them for new.

Prismatic troffers are quite rare now here in the UK (I believe I have also seen them called modulars here?) due to the prevalence and low cost of drop-in LED troffer/panel replacements. I cannot comment on the original popularity of prismatic troffers here in a time when they would've been installed new, as I was not alive then. Nor can I comment on how common dropped ceilings were in such buildings, however I imagine it is less than in the US - perhaps a lot less/no air conditioning negated the need for dropped ceilings back then in the UK.

I imagine in the US you will find older troffers much easier than here - good luck finding vintage and exciting fixtures!