If the gasket you need is the big one on the back, and I can't think of any others, then you could try making one. Take the cover off, clean up the mounting faces, and lay a piece of thinnish card on the diff part, as opposed to the cover. I've used old corn-flake packet type stuff before, works a treat, stiff but slightly compressable, Lay the card on and tap
gently around it using your lightest hammer or a round-ended (ring) spanner, only need to hit it hard enough to put a crease in the paper, then borrow some small sharp scissors (best to use someone elses

) and the mounting holes you can do with the small hammer or small spanner. May be best to do the mounting holes first, put a couple of bolts in and they'll hold the paper still. Easier to do than explain, especially from a distance, but if you practice a bit first you'll soon get the hang of it, and it'll save a few quid and a bit of waiting/collecting from wherever you'd have to go to buy one.
Done this myself many times, and it works, even used to get rolls of gasket paper back when I were a lad, not seen that sort of thing for a long time.