I don't think you have to have it calibrated. The MRB625 Wein Cells are 1.35 V and the same size as the mercury PX625. Voltage and size the same. The discharge curve is flat, but the life is a bit shorter. Whenever I am not going to be using my Luna Pro, I take the batteries out and tape the holes shut which stops the chemical action and lengthens battery life. I generally get the same life out of Wein Cell (Zinc Air) batteries in this manner.
The Wein Cells are sold by a number of battery purveyors on eBay. I think they are about $50 and I get mine delivered in 3-5 days.
Another option may be to find Gossen Luna Pro with the SBC (Silicon Blue Cell). That cell reads in lower light conditions, expanding the range of usage of the meter. It also uses a conventional, readily available, 9 volt duracell or other.
I just did a quick look on eBay and there are 3 SBC Gossen Luna Pros up right now. I just searched luna pro sbc