Can’t you lift the joists one end to get the desired pitch by added a timber packing piece to the plate ? Then all you need to do is plumb cuts either end.
Otherwise get joists sufficiently deep enough to allow you reduce them by whatever you need one end. Or you can cut a length of timber in two to get 2 firring pieces out of one, and then nail them on the joists, but as they’re only short lengths it should be easier to reduce the joists.
Otherwise a decent timber merchants should be able to cut firrings for you, although I expect it will be pricey (I don’t buy materials)
Example : http://www.applegate.co.uk/all-industry/products/firring-pieces-cut-size-0103406.htm