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.
If you want mylar, you may be able to find it at your local sporting goods store. (This is the same stuff that gpoehlein was just talking about.) Hunters sometimes carry a "space blanket" in their emergency supplies in case they get lost or stuck somewhere in the woods for the night. The space blanket is a large sheet of aluminized mylar sheet about six feet by eight feet (two meters by 2 & 1/2?) for a few dollars. That is about as cheap as you are going to find mylar in your own town. They call it "space blanket" because it is shiny and high-tech-looking, and pretty light weight.
Do you remember seeing any products like that in your area?