I recently built up a bike for a friend with a tapered frame and straight steerer fork.
Tapered fork to non tapered frame adapter.
Fitted the headset and the adaptor crown race to the fork and works perfect.
I got a giant integrated tapered 1 1 8 1 1 2 headset from the lbs which also had the straight steerer crown race adaptor in it.
How to install a tapered fork into a no tapered head tube using hope pick n mix head set cups music by.
If your mountain bike came with a straight steerer tube you can use this bearing adapter to put in a tapered steering tube and run a different fork.