ive worked with sintra extensively and ive never experienced the fume problem DURING HEATING. but, i've never cooked it either. ive heard that is te worst method for fume production but the best for quick easy shaping. i figure i dont wat killer fumes where i cook my food. for 3mm sintra, i use a heatgun for shaping and it works like a champ. it's basically a hair dryer on steroids you can get at a hardware store for about 15.00. 6mm sintra is a beast. i have an old deep stock pot i use. i bring the water to a boil and drop the plates in. ive not experienced fumes with either method. as a side note, i'll never prepare food in that pot again cuz who knows what residue is a part of it now.
ive only experience fumes with sintra when cutting it. i also cut it in the garage with the roll up open or outside on the patio and i always where a cheap respirator mask. 3,,can be cut easily with a dremel. 6mm will kill dremels like raid kills bugs. for the 6mm you need to use a skill saw.
as far as getting it... sintra is a name brand for pvcx foam board like kleenex is a name brand for tissues. when you go hunting for it, ask for it by both names. it's primary use is for sign making. if you have a store nearby that makes custom signs, you can hit them up for it. some but very few hardware stores will have it. also, plastics suppliers carry it as well. prices will vary by a couple bucks here and there depending on where you're getting it but a 4'x8' sheet of sintra will make 2-2 1/2 full sets of light or boba style armor or it will make 1 1/2-2 sets of medium or jango style armor... minus the gaunts. the sheet of 3mm sintra will cost about 30.00. a sheet of 6mm in silver grey (my personal favorite) will run about 75.00. if you dont have a local source and have to have it shipped in, by the time you buy it, pay the shipping, print the templates, cut it and shape it, youre better off buying a prefabbed set. tk2280 here and at mando mercs makes very nice very fair priced sets. also cyanide on ebay has nice sets as well.