FLUX Store Filament page
You may need to flush browser cache? Or just try reloading page? Here's the link to the store, it just opened a couple days ago, and filament is on sale right now too.
You can get filament from third-party suppliers although FLUX recommends that you only use official filament and it could possibly cause issues with warranty.
If you are going to use what are commonly known as more abrasive filaments like metal-fills or wood-fills, and plan to start changing nozzles, just keep warranty considerations in mind too. Those filaments do wear out nozzles, though it is quite easy to change if done properly.
Not trying to discourage, just want you to be aware of what you're getting into. As far as I know, FLUX has never said anything about third-party filaments other than that recommendation way back at the beginning. They've even participated in the discussions here, and heck, they've improved the extruder so we can use flexible filament which they don't have, so...
As far as nozzles, you can go as high as you want (Ruby anyone?) but for the price of those hardened, steel, tungsten, ruby etc. nozzles you can buy dozens if not hundreds of regular nozzles. I have used .3 regularly and gone down to .2 nozzles as well. It prints like a dream with those too, but print time is tremendously slow.
Several threads about this if you search, and lots of supplier links have been posted if you are looking for someone local you might find one too.