For regular Simple Green, the label reads;
Water, surfactant, chelator, builders, colorant and fragrance.
The data sheet I found says it has;
Water, Ethoxylated Alcohol, Sodium Citrate, Tetrasodium N,N-bis(carboxymethyl)-L-glutamate, Sodium Carbonate, Citric Acid, Isothiazolinone mixture, Fragrance, Colorant
There is a place called ISL Industrial Limited that looks like they are in New Zealand (warehouse/office facility located in East Tamaki, Auckland) and sells Simple Green. A local supplier might make shipping more affordable if you don't live close enough to visit their office.
Here is a link to their Simple Green page. I would probably go with SG35042 "SIMPLE GREEN OUTDOOR CLEANER 5L CONCENTRATE** MAKES 10L (50/50 MIX)" for $23.00. The jug I've got is 3.78L and it has lasted a few years with maybe 1/4th left. Note: I live in the States and have never had any dealings with this company, so I can't testify how good they are to work with.
What I can attest to is how good Simple Green works... I tend to use paint remover to remove glue, and Green Stuff to remove paint... As for how gentle it is. I found a jar with minis in it about three months back. I use the word "found" because I completely forgot about them. I bought the minis from a private individual in June of 2015, and they had probably been in that jar since then. I didn't actually take them out until last week. Mainly metal, there was one of the old PlasTech minis in there and it shows no signs of having been harmed after being submerged for a year. The metal ones seem not to have been adversely affected either.
Things to know about Simple Green... If you are looking to strip paint in ten minutes or less, then this is not the product for you. Anything that works that fast will eat a plastic mini too... You need to let them sit a couple of days, and it may take more than one dunking.
And finally, if ISL doesn't work out for you, I have heard that Pine Sol also works on plastic minis. But what I have read is that you have to be a little more careful and not leave them in too long. Again, I have no experience using it, but I thought it might help to know there are options.
Good Luck,
Caz