as you are close to Woodlands I would go there first. On the other side of the city is arnold laver and pretty close by to them is panel depo ( the clue is in the name but I am prety sure they only do sheet goods but they can prety much sell any sheet good you can think of definitely worth a visit)
I think selco near Sheepscar should have one in stock or you could try Woodlands over by Horsforth. I used to be in Leeds and I definitely now miss having decent lumber yards and interesting builders merchants so close to hand.