Where to buy thick door knobs?

I know somebody posted a door handle extender the other day, but I can't remember what thread it was in. I remember they said it was expensive though.

It seems like most people are using automatic door seals on the bottom and hydraulic closers on the top, with a push plate on one side and a stationary handle on the other. I mention that so you know that a door knob isn't the only option.

Can you explain your plan a bit more?
Thicker doors are normal but not that thick. Is it for style reasons?

Would communicating doors be easier? (Communicating doors probably have better performance than a single thick)

When doors swing open they are longer on the diagonal than when straight. This will be even more pronounced for a thick door so you will probably have a gap or a bigger stop in the frame.

here are 141 door levers for doors up to 3 inches thick. Door Hardware, Door Knobs & Door Locks

Thank u all for getting back to me. Fresh out of the er. So I custom ordered a metal door 22ga 36.5 wide by 78 tall. Made for a 2 x 4 frame. Solid core. The wall on the other hand has been way over built. Outer side has 2 5/8 fire rated 72 pound each 1 1/2 mold rated 54 pound sheets. 1 6/8 than the inside room is hushframe decoupled system with 2 5/8 with green glue. 1 2/8 thick. So I don't think I can fit two doors there. My metal door is 1 3/4 thick stock. Plan was to add 3/4 mdf on both sides and 1/2 inch mold rated drywall with green glue between all the sheets. Plus build a new seal system to go with it. I need to be able to lock this door is the reason looking for bigger door knobs. Wife will freak out if I do something nasty here.

If you mean bigger as in the additional thickness, that should be easy. If you are looking for a common home door lock like Schlage or Kwickset, they use a 1/4" square stock to connect the knobs to the latch. You can find some square stock at any hardware store and cut to required length, you will have to buy longer screws too.