Black out curtain fabric recs please

[RunsWithScissors]

Need to line some blinds I'm making. Would like to use black out fabric that is washable (dustmite allergies= lots of laundering of the blinds).

Anyone have a good site I can buy from, not too pricey would be great, as will be making two sets.

Thank you!

They sell it in John Lewis. You pay by the metre, not v expensive. I tried to line blinds with it but it didn't work very well though as it made the blind too thick to roll up properly, but maybe you'll get it work better.

Thanks! I'm doing a variation of a roman blind (instead of cord to pull up, using buttons and loops to button the blind up to varying levels, if that makes sense).