Buying my preferred breakfast has always proven somewhat difficult when I'm not in my country of birth, Switzerland: if I'm asking for "cereals" then I'll be presented with a shelf full of processed foods as described in this article but not what I'm looking for, when I'm asking for "oat flakes" then I'm presented with some kind of "instant oats" that's meant to be eaten hot, whereas I'm eating mine cold with milk and fruit as traditional Swiss muesli [1], and instant oats are quite disgusting when used for that purpose (mushy, and I think even the taste is worse). Very often shops actually carry some muesli mix, but those are always many times more expensive than just buying oat flakes (I gather that the right term might be "rolled oats") and mixing them with fruit (and optionally other kinds of grains, nuts etc.) yourself, and almost always I don't like them as they don't actually contain oats but wheat, contain more dried fruit than actual cereals, or have them roasted and mixed with sugar which makes them crunchy (some people seem to like that) but unhealthy and tends to hurt your gums. I find it quite perverse how forces are set up towards extracting money from you for worse health and, in my honest opinion, worse taste [2].

Even in Switzerland those mixes are getting an ever stronger foothold, but you can still buy bare rolled oats easily everywhere. In Canada and England you generally can't buy them in the standard supermarket except if you're lucky, but you can get them from health food stores, also some unexpected places like "Jamaican Oats" from a Jamaican food store. Some kinds found in alternative places are the inverse of mushy: pretty hard and with a tendency to be bitter, they are still usable if soaked over night though. So, it takes a bit of time to find the right source. There are smaller and bigger variants, this doesn't really matter (other than giving a different texture).

I'm preparing them with zero added sugar (if you're used to sweet cereals you may want to start with sugar and lower it over time), just oats, milk, yoghurt, fresh fruit, sometimes a small amount of raisins, and nuts, let them soak for a bit (at least about 15 minutes, e.g. while eating some bread, to ensure they are not dry anymore inside), and for guests add some cream for even better taste :). You can find various "muesli recipe"s online that are variations of this.

[1] https://en.wikipedia.org/wiki/Muesli

[2] and it's not just subjective, it seems: I've prepared muesli for a Canadian and he found it delicious

PS. I use whole milk. Skimmed milk is crazy, way worse taste and probably worse for health (since, as seems to be the consensus nowadays, it's not fat that makes you fat [3]). Also, yoghurt made from whole milk and with no other ingredients (i.e. nature, no sugar or fruit or artificial flavoring).

[3] anecdotal evidence from myself: I'm slim, but put on weight when moving from Switzerland to Canada, but that stopped and reverted after I made an effort to find the food that I was used to, which involved finding oats for muesli, buying bread made from whole grains, (raw) sauerkraut (can be eaten raw with olive oil), etc. (I also felt stressed at that time, which may have had an effect as well. But one effect that muesli clearly had is that it relieved me from problems with constipation.)