Synthetics will not breathe and this is a big problem when fighting, especially in armour. Stay away from fabrics that have a shiny surface finish. They look very modern and often cheap, although it is possible to get rid of some surface finishes with a good wash.
Raw silk can often be found at reasonable prices ($5-$7/metre), and can be a good substitute for wools. It looks like roughly woven cloth, will take and hold dyes well, is light and breathes very well and best of all can be thrown in the washing machine over and over again. (Despite what the manufacturer’s instructions say!).
Not recommended are fabrics or braids that you can’t wash in a washing machine, especially if you are going to fight in them. Not only will you sweat heaps but metal armour gives tunics all sorts of disgusting colours and smells! Upholstery fabrics are always worth a look. They can be very cheap if it’s a roll end and can give an authentic looking “rough weave”. Beware for they can be very stiff and scratchy but a wash in detergent with some wool mix or fabric softener can do wonders.
Small point: ALWAYS WASH YOUR FABRIC BEFORE SEWING, DYEING OR EVEN CUTTING ANYTHING OUT. Not only will colours run and the cloth shrink, but the cloth can also shrink at different rates in different directions (I have a pair of pants that fit me everywhere else, but are now 6 inches too short). It’s also a good idea to wash your braids before sewing them on. Another point on washing, always treat fabric softeners with respect. These products soften your cloth by causing a partial unravelling of the tightly wound fibres. This can be great for softening harsh fabrics with an initial wash BUT if you continue using a softener the fibres will continue to unravel causing weakness and probably a premature demise of your costume.
It is best to stay away from materials with applied patterns, (i.e. dyed or batik) – stick to those with a plain weave or very basic woven patterns.