No fast film available? There is another way to get the high ISO medium you desire and that is by pushing a slower film instead.
Before we start on the topic of push processing, a word about the faster films that use Kodak's Vision movie film technology. Portra 400 and CineStills 800T use Vision 3 and Portra 800 the earlier Vision 2 technologies.
Both these technologies impart these films with a very wide exposure latitude (V3 being wider than V2 as you might expect) so much so that I regularly expose Portra 800 at 1600 when needed with no ill effects and no special processing and Portra 400 at 800 and at a real pinch 1600.
Acceptable exposure latitude for Portra 400 runs from 50 to 800 ISO to my tastes and for Portra 800 from 320 to 1600 ISO without any special processing. In practice actual underexposure for both films is much wider than this, I just don't like the reduction in sharpness and shifts in colours you get, but this is personal taste thing.
With CineStill 800T I don't a lot of experience, but the makers state that you can rate it anywhere between ISO 200-1600 for acceptable exposure.
So now we come to push processing. Pushing is where you expose a film a stop or more higher than its box speed and extend the processing time to correct for the underexposure. This typically causes an increase in film contrast, grain plus a colour shift all of which, if you push this too far (pardon the unintentional pun) typically beyond +2 stops, can suddenly become extreme.
But with Portra 400 you can push process +1 stop and expose up to 3200 ISO and still get excellent results. No wonder this has become the go to film for so many of today's stills film photogs. It's a bit of a wonder emulsion. I have never pushed Portra 800 but simply use it at 1600 or indeed at a pinch 3200 which works really well for me.
For 800T, CineStill state that a 1 stop push can be used up to 2000 and a 2 stop push up to 3200. So push processing is a really great solution to creating a high ISO film equivalent.
There are practical downsides to a push processing. One is finding a Lab that has the capability to do this, they are few and far between, then there is the added expense. There is also the fact that these days most modern professional colour negative films have about 2 and a bit stops of under exposure latitude anyway so don't need pushing anyway..
Finally, you have to rate the whole film at the chosen speed and push process all of it, you can't change speeds part way through the film as you can with say with Portra 800 which I will rate at 800, 1600 or even 3200 (without pushing) on the same roll as the situation demands.
These days I don't push process because of the downsides, but use both Portra 400 and 800 films which I will rate at 800 and 1600 respectively if needed. I will also use 800 at 3200 and 400 at 1600 in extremis depending which I have got loaded in the camera at the time. I would not try this with films without Vision technology, however. You just have watch your exposures and meter for the mid-tones.