Cilantro is widely available in the UK, but we call it Coriander. The botanical name is Coriandrum Sativum, and it is also known as Chinese parsley.

Asian food shops are likely to have it, but so do many 'ordinary' grocers, supermarkets, and even veg markets.

I could imagine ground coriander seed going into pepperoni, but not the herby tops, which are the bit which (imo) taste soapy. I have no idea what has caused your problem (unless the Coriander guess is right) but it sounds unpleasant. Hope you get no lasting effects.