my gym changed treadmills last year from a model that had topspeed of 12mph to the current ones which like you, have a 10mph top.
I've had a look into buying my own, and surprisingly a large number (at the cheaper end) have a 10mph topspeed
Rather than fork out 2k+ I would opt for more reps.
I tend to do up to 10 x 800s at 10mph, with 400 recovs.
I however alternate this with 16 x 400s with 200m recovs, 6-8 1200s with 400 recovs and 5-6 1600s with 400s. The latter two are quite tough at 10mph....
For sharper sprint training, I often do a 20 x 1min fast / 1min slow on the roads. I try to run quite hard but not 'flat out'.
The other alternative is a football pitch/park, doing 10-15 laps, sprinting the 'length' , slow jog/walk behind the goals.
If you have access to a track, then timed 200s/400s are also an option, but getting the correct pace (say 84s 400s) is a matter of trial and error (or it was when I tried!)
Like you, I find the treadmill works well given its limits, hence the alternatives.
Hope that helps.