Knowing the backend of the ink industry is somewhat of a secretive area of expertise lol. I have worked with several of the ink manufacturers, but they're all very hush-hush. Polymeric IS an OEM supplier for several brands of ink. The patents are held by the OEM's now and they're under an NDA to have no clue what you're talking about if you ask them. If you just call them up out of the blue an ask to buy ink from them, I'd be very surprised if they would let you. I'm strapped by several NDA's and respectful relationships with some of the main suppliers, so there's not much I can really give out in detail to be honest. If you're REALLY serious about going 3rd party inks, you can find the information if you talk to industry people and keep your eyes and ears open. Not sure which flatbed you have, but if it has RFID bags then you need to sort out that too, or go with a 3rd party option that gives it to you turn-key.
To be honest, buying 3rd party is often much less of a savings than most people think. It is certainly good in some cases, just not all. Mostly because if you're running enough volume to really save $20-30-40k on ink, then you should be getting the best Vutek/Dhurst inks at less than $80/liter. People running little HP's/Latex that are tired of paying $130/liter ... well they dug their own graves with that sorry, HP isn't into reasonable prices just like they're not into reasonable service. If you're getting bulk ink pricing at $70/liter, then is moving to 3rd party really worth it at $35/liter? Sure you save money, but at what cost? All inks are CERTAINLY not created equally. Knowing what you're getting is the biggest unknown in that industry, ironically enough.
For turnkey solutions Nazdar makes good inks - I've used their screen print and digital inks for years with no issue. Always been super impressed with their customer support.
Bordeaux makes good inks too, used a bunch of them. They can be a little difficult to reach since they don't have a dedicated "old school" sales staff. But you can certainly place your orders and receive them simple enough.
I have no experience using INX inks, mostly because their old VP/Sales Manager was somewhat of a dumb-a**. They're still on my boycott list.
Basically, it can be done, but you really need to know what you're doing to do it with minimal risk, and you need to be sure the risk you're incurring is worth it. Sooner or later you will have to write a check for something you might have avoided because of the inks you're using, so make sure it's a net profit and not a net loss
for the information so far.