Two suggestions: Coles Online, and: know that you are not alone.

I became a screamer with my third child. Possibly it was because of my thyroid condition (makes me feel like I have constant PMT) possibly because this child just really knows how to push my buttons. I find that it helps to remind yourself that so many mums feel at a total loss about what to do too and that no one of any worth is judging you. Also, you know what? Don't feel pressured that you have a half full trolly and need to complete the shop and pay. Very few people would even notice if you just parked the trolly and walked out

Oh and also... if you can bite your tongue for the count of 10 the urge to scream often passes. I visualise flicking a switch... switching off the banshee mummy button