The IMU is part of a tracking module component called the T150 Tracking Module. This can be attached to the D4 Vision Processor component of the camera's circuit board module via a 50-pin connector. It is only validated for use with the caseless D430 circuit board module version of the camera, not the cased D435 USB model (since a T150 tracking module could not be added without opening the D435 camera casing and invalidating its warranty).
You can read about the Tracking Module T150 and its IMU component in the camera's data sheet document from page 41 onwards.
I do not have information on how this separate T150 Tracking Module component can be ordered yet.
MartyG already shared some information with you. I would like to add some information too.
If you refer to old datasheet, D430 was supposed to contain Tracking Module.
D435 was supposed to be D430 + RGB sensor. So technically D435 should have tracking module. In turn, tracking module is actually Fisheye camera + IMU.
I had an old D430 sample, where both Fish-eye and IMU data worked. This was with librealsense 2.6. Once I upgraded librealsense to 2.8, then IMU didn't work any more. In fact most of the rs apps just crashed. This was later fixed by a new firmware for D430 where Tracking module support was disabled. Intel support didn't cite any reason for disabling the support. They just said that in case we rely on tracking module then this will be enabled in one of the future firmware upgrades.
I am not sure what was the reason reason behind disabling it. I can think of several things
1. Not many users practically needing this feature
2. Design issues due to tracking module consuming additional power and making camera unstable
3. Data throughput issues
4. Any thing else
Now if you look at D435, then there is no Fish-eye camera slot. Tracking module is Fisheye + IMU, so I will assume that this module is physically missing from D435. Probably including this in design made camera more bulky, and not many customers required it. TM150 is certainly still mentioned in datasheet. I wonder if Intel will later plan another version which will be current D435 + TM150. One thing we know is that Intel has been swamped by interest it received on D4xx cameras. They are low on supply and there is high demand. Once backlog clears up, then we may have some clarity on future releases