If at all possible go for the genuine Brother mylar sheets, whatever the cost, even if you can only afford one. I have five mylar sheets which are over 45 years old and have carried at least 200 or 300 shapes each and probably far more. Having had their shapes drawn with washable (watercolour) pens each time, washed away with weak washing up liquid solution, kept rolled in their original container when not in use, they are all still in perfect condition, not even discoloured and certainly not brittle.
Hope this helps, for all looking for knitting machine stuff.
the site is Hong Kong Knitting machine parts. contact Sunny Choi, they have a wide variety of products , are very quick with delivery, offer free shipping. I have seen quite a few pattern sheets on there so may be good to try them