Chris,
The MB part no. for the citric acid is 00-989-10-25. That's for 0.5kg(500g) or 1.1 lb. Cost $17.
If you are going to flush your cooling system, you really need to use a degreaser first. The acid will not have any effect if grease/oily deposits are in the system. The degreaser is part no.001-986-21-71 for 500g or 1.1 lb.Cost $11. You can find the citric acid in your Super Market. Look in the home canning section. For the degreaser you can use a non-foaming automatic dishwasher compound. Do not use strong alkaline cleaners on light alloy(aluminium) radiators.
Bill Lewallen Lexington,Ky. The Bluegrass State, but the grass is really green, and we use the GREEN coolant.