Dog coats can be expensive especially as your best to have at least 2 per dog, so you can wash one or let it dry from a previous walk. A cheap way I have found to make dog coats is to buy a second hand kids jacket from eBay/charity shops etc. My staffies need age 5-6. when you have the kids jacket you need to cut the arms off and hem the edges of the arm hole this will be for your dogs legs to fit through. If you wish you can leave a bit of the arm so your dogs legs are covered a bit. Then you remove the zip and hem the edges and sewing a couple of strips of velcro. I then cut a slit on the back of the jacket for the harness d ring to fit through, you can do the same higher up at the back of the neck to fit the collar through. I then cut off the bottom of one of the arms I had previously removed and hemmed each end then made a button hole at either end. I sewed 2 buttons onto the area where the stomach would be so that I could then attach the bit of arm, this is to cover the belly as dogs tend to have less hair on this area and Tess is completely bald!
The same can be done with adult body warmers and for the body warmer I added a nylon belt that snaps around the dogs waist to keep the jacket in place. For this I used an old harness that they had outgrown. On both I finished them off with a length of reflective ribbon