I went to WHSmith and bought a card index box and several packets of 6 inch by 4 inch record cards. they don't have them online, but I am sure everyone knows what these things look like.
I was briefly excited when I noticed they had some coloured pastel cards as well - yellow/green/pink/blue I think but then I thought that would subtract from the colour of the swatches and it was best to stick with plain white, if a little boring.
So the idea is to cut a swatch from each fabric, staple it to the card and write on the card
colour and type of fabric, fibre content if I can tell, size of the piece ie 150cm wide, 3.5 m and any other stuff. Most of mine I can't recall what I paid for it, and not always where I bought it though I will start to add this for new stuff going forwards.
I have not started yet, so we'll have to see if it comes to anything after all.