You might think that in France with its haute couture reputation, dress fabric shops would be all over - they're not, and what there is is very expensive.
So I buy mostly online, but there are some problems. eg the colour or texture isn't always exactly the same as it is in the advert.
I bought some needlecord a couple of years ago from a company in the USA. They have a fantastic range, many types of fabric you never see in the UK, very good prices, but the postage was more than the cost of the fabric.
People have commented on the disappointing range in John Lewis - I thought that too last time I was there.