I'd just go the John Lewis and see what you like - needs to be durable (so not linen or anything light, plus no obvious woven pattern that can get pulled), check cleaning advice etc. The fabric books are expense to buy (fabric houses don't give away for free) which is why your upholsterer likely won't have any, but they'll still be able to get trade ££s I'd have thought - whether they pass those savings on is another matter....
They should also be able to tell you, roughly, how much fabric you'll need based on a plain only, and if it's a pattern then it'll be more. So you can have a guesstimate of fabric costs as you look around.
And always get them to buy the quantity that they need, but you should sign off what they buy on your behalf. If they ask you to buy it, then if they find anything wrong with the quantity, it'll be your fault.....you need to push that ownership onto them.
Get 2 swatches of what you like, one for you to check that they have bought the right thing thing, and one for them to check they are buying the right thing....
Think that's all the basics.....