
How about looking in a charity shop for a cheapy one, taking it apart and using the pieces as a guide, if you can't find a pre-done pattern? Just as a back-up plan.
When I made Remy's coat I used him as the pattern - I used some old sheets cut into fair size pieces, then had him stand while I laid them on him and marked the shape with a marker pen. Took a bit of adjusting and I made a rough coat in the sheet first, but it worked a treat - he's coat a coat now that goes down (and round) his legs to his elbows/knees and under his belly, it zips along the back. Lovely fit, much better than anything shop-bought - dobes' deep chests make buying coats that fit well dang hard! And, they don't take into account baldiness :p