The typical culprit for poor hip mobility with squatting is either tight hip flexors or the deep hip external rotators.
For the flexors, I'd recommend a hip flexor stretch
If kneeling is painful, apply a pillow under the knee or perform it in standing (with a wider stride length and ensuring you are going into a posterior pelvic tilt).
For the ERs, I'd recommend a modified piriformis stretch like "stretch #2" featured here:
Stretches 2-4 are actually all great ones for hitting your hip extensors and external rotators.