I think there's no need to buy any kind of something "very special". That foam you can get from cans for internal cleaning of cars. I have one made in the US
Sorry, I haven't posted a picture.
The volume of the can is slightly bigger than "magical" fx can. The only difference, you'd have to search for spray heads as can for cleaning comes with one type of spray head, but it's possible to find and to use another head from other cans, certainly it that head fits your can. Having different heads you can experiment with the pattern it produces. Besides, you can make custom made head, I don't think there's something special there.
I also tried shaving foam. But again, you need to have another spray head, as in the case of shaving foam you just can not have any spraying at all, those using shaving foam know what I mean. I've sprayed this foam onto white paper and being removed after drying it leaves nothing on the surface. At least I've seen and felt nothing. So, you could experiment with it also.
It would be great from someone having Grunge FX can to put here photo of the ingredients of the can (shown on the backside). I could take a picture of my can and we could compare the ingredients of both cans to find out if there's any difference. I've paid for the cleaning can some kind ridiculous price compared to "original" fx can. I mean I've paid that price locally, so I think for people from the US it could cost even less, not saying the price for "fx" spray is not something I'd pay for, especially if you experimenting for yourself. One thing if client pays for it, another if you do.
Mind, that foams do not work with waterbased paints.
Highlights should be done by using scratching and erasing techniques, it's much faster and effective than using an airbrush for it. You will not have any colour shift and if the highlight is too bright, you can always dull it using your dark colour.