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There is a typist just opposite Mercur Grand (Sofitel), opposite the laundry, next to the juice stall/cafeteria. He has all letters preformatted. Just give him the details and he'll fill it in. Charges Qr 5 for a letter.
I've gotten all my documents from that place. Haven't had a problem.
Easy way is get information of your both sponsor.do not forget to keep information of your passport. and go to immigration(main), there are lot of typists. anybody will draft a letter for you