Easy:
* Red lorry, yellow lorry.
* Round and round the rugged rock the ragged rascal ran.
* Rubber baby buggy bumpers.
Medium:
* Rita's red ribbon ruined Roger's racing record.
* Raging rivers roar and rumble, rocking rugged rocks.
* Really right red robin, resting in the rain.
Difficult:
* Roy Rogers rode a rapidly roping red roan.
* The river rippled, rushing rapidly, right round the rugged rock.
* Three grey geese in the green grass grazing, grey were the geese and green was the grass.
Let me know if you'd like more! I can create some custom tongue twisters starting with "R" if you have a theme in mind.