Hi Kate, well LBS is the cheapest I know and offer a good range of sizes, also a thicker fleece at 30gsm for winter use, but I found it strangely fragile and less enduring than the standard 25gsm.
Not all plantings have to be under fleece now, it depends if you have rabbits and perhaps how soon you want to pick leaves. I shall be planting spring cabbage and onion this w.e without a fleece cover, just netting over cabbage against pigeons. I shall cover some newly planted mizuna to help it establish, may remove the fleece mid October if it is mild to let some extra light in