I used to just gate the heck out of the soundtrack whilst trying to keep any useful dialog's head just above the water. Start by hard-cutting all the bits that don't have dialog.
I used to use movie trailers straight from YouTube, rather than try to find bits of full movies I could re-score. You kind of have to pick ones you think you'll get the best separation from, rather than just go for your favourite movie. It leaves the choice rather eclectic & probably more of a musical challenge.
If you mix the dialog low & the score high, you kind of get away with it.
I wasn't hugely successful at this, but it was fun to try -
These days you might get luckier & the soundtrack is in 5.1 - giving you a much simpler way to pull out the centre channel.
Three examples - quite old now, so the videos are pretty low quality; not really important in the overall scheme of things.
This first one had a lot of dialogue, some of which I managed to keep intelligible, some… not so much, but it gets the feel across, which is really all you need.
This one got the same treatment, but had cast singing in one part, so I incorporated that in my score.
This one had a voice-over, no dialog, but was impossible to separate from the original score, so I re-did it myself.
I wouldn't claim any of these were great, but I enjoyed doing them ;)
Another thing you could do is hook up with amateur or student film-makers. Many of them are in need of scoring for their movies - usually shorts - so you can usually knock out the first draft in a day, finalise the next.