Is it the
knee lever itself or the
porcelain part placed in the smaller compartment to the right in the base? Either can be found on places like fleabay and with a bit of luck else where too. The lever for model 99 is a bit shorter than for full size machines like 201 or 66, they will work with the machine but will not fit in the case. If a 28 or 128 is fitted with knee lever control, it should be the same as for the 99. I'm sure you are aware of it, but just in case; the knee levers fitted in cabinets are different from the portable cases.