My little Buster is on a low calcium diet. He has the Vetcare Urinary Tract Formula pellets, one handful twice daily. The pellets are really long pieces so it's good for teeth and they are high in fibre (Timothy Based) so good for tummies as well. I order mine from the Vet UK website, £5 per box but lasts a month for one bunny.
Also important to remember to feed low calcium veggies. I feed celery and cucumber in the main, but also small amounts of peppers, apple and carrot. They tend to have celery and cucumber 3x weekly, Saturdays they are allowed a small piece of carrot or apple for a treat. Even though broccoli and spring greens are higher in calcium they can be given now and again as the calcium in veggies is metabolised differently to that in pellets. They maybe get these once or twice a month (small portion). Good luck x