Mainly kill monsters that drop them. Dont know exacly which ones do unfortunatelly.
Kill drowned dead, they drop red mutagen 1/3 times. Run around the Skellige coast looking for them. Another option is the mansion you get in blood and wine, your herb garden here will grow winter cherries. You can use these to make red mutagen, no monsters needed.
Get Blood and Wine dlc.
You don't have to wait till you go to Skellige OR play Blood and Wine. Alghouls drop red mutagens, I got like 3 or 4 of them already and I haven't went to Skellige yet. I forgot where I found alghouls but I'm pretty sure that they were in a quest. Just find them and kill enough, you will get red mutagens I promise.
Pretty difficult to find during my first playthrough. I was completing my mutagen tree and hell it was hard to find them through monsters, although you will find the green and blue mutagens in abundance.
You can also find them by splitting the monster specific mutagens (if you reached that phase already) but keep in mind to check the certain decoctions first as they will be needed to create them.
In velen, try Toderas village. It used to run by bandits, kill them first. Go to other continents, then alghoul and some exploding monster will appear. Kill, go to WO, come back. Repeat.
What do you think?