"Disc" makes me think angle grinder, I've never heard of a 150mm angle grinder so I'm not suprised discs are hard to come by. If it's a bench grinder, try "grinding wheel".
I think that size of the machine is rare in UK. We go 4, 4 1/2, 5, 7, 9, 12, 14. I think the Americans use 6" but you would have to confirm the centre bore is correct.
I have two 125mm Metabo grinders, the discs are a lot cheaper and more available than the 150mm ones so I think I would be using them for the easiest solution.
Do Metabo do a 7" version of this? If so, could you buy a 7" guard as a spare part from them (or even a 7" Makita guard), fit it and use 7" disks?
I suppose it would not be in the interests of the people who sell these machines to inform potential customers that consumables for them are rather difficult to obtain.
Was able to find some but now I can’t find any anywhere now. Awesome grinder don’t think you could buy a better one and I’ve tried most. Why they’re not popular over here I don’t know, literally has the power of 9 and disc life but without the weight and vibration of them. Miles better than a 4.5 or 5 but now I’m stuck using worn down 9’s or 5’s