In our grocery store all fruit etc are packed in plastic bags that are really, really thin - maybe half the thickness of a garbage bag. Is that what you're talking about or something thicker?Mylar is nice, but it is not necessary.
There have been zillions of kits built with plain old plastic parachutes. This is the standard material for many manufacturers, especially the big companies that sell 99% of the kits. Plastic sheet works fine, is plenty strong, and is available just about anywhere. You can use plastic shopping bags from the grocery store (if they even use them in your country, but that's a whole different discussion), from other local stores, or salvaged from other uses. If you have "party" supply stores in your area you can often find large plastic table covers in a variety of bright colors; these table covers are usually very inexpensive.
If you have thin plastic bags from your local clothing dry cleaners, this too can be used for parachute material. I used to use dry cleaner bags all the time for competition rockets, but this plastic is not as durable. It is so thin and soft that it stretches and tears easily, and if the ejection charges get past the protective wadding, dry cleaner plastic will melt quickly. These parachutes are usually only good for a few flights before they become damaged or unusable.
Do you remember seeing any products like that in your area?
Hmm, black thin garbage bags. We have loads of them, that should do.Space Blanket? I use that when I want aluminized mylar - only about $2 at Walmart. I don't know what you have "across the pond", but hopefully something similar. Space Blankets are a bit heavier gauge than parachute mylar, but not bad if you are building for a sport model.
For contest use, I tend to buy the cheapest garbage bags I can find. One local source has black 30gal garbage bags that are 0.55mil thick, which is great for contest duration chutes. They "jellyfish" quite nicely in a thermal.