Ummm, not sure why you would want to cut your calories to only 500 per day? That's not too good for your health.
The Optifast diet is a VLCD (very low calorie diet) & was originally designed for people in the obese category that needed to drop the weight quickly before surgery. I'm pretty sure that it only amounted to 800cals per day. I'm not too sure if it is still used for this but there are alot of meal replacement or "shake" programs on the market nowadays which you can buy online. Celebrityslim, Tony Ferguson, Betty Baxter, Dr Tim's, Kate Morgan just to name a few.
Is there a specific reason why you do not wish to purchase the optifast shakes at a chemist? Some chemist's will weigh you & determine whether or not you fit the criteria for a VLCD. As I mentioned before, 500cals or even 800cals per day is very low & not really recommended for sustainable weightloss &/or wellbeing.