These are actually Tacony knockoffs and they were made to be competitive to other companies like Tacony (Riccar/Simplicity), Matsushita (Panasonic), Sharp, and Sanyo.
The reason why parts are hard to get is because the 6600 was discontinued years ago. You can still find basic parts like the hose, brushroll, attachments, or the handle for example. But other parts like the dust cover, main nozzle, or any part that is white from your 6600, they are really hard to find. I highly suggest look into at your local vacuum stores.
I don't fully agree with Tacony making better clean air upright vacuums than ESSCO, there are some things that I like better. They have a bag safety switch where it will not run without a bag, they also shut the brushroll off in upright position, I like their floor selector switch better because you can use your foot without requiring bending over to switch from carpet to floors, I also like that it can store extra attachments on-board, I think it's unique on some models where it has a speed selector, and these are built and they can actually clean just as well as Tacony but definitely cheaper.