"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.