Hello,
Ash is good bow wood most of the time. If you have bad ash (which is light, tight ringed and somewhat porous), you have to make it longer and larger. If you are working from board, get straight grained board (red oak, ash, hickory). Just take the heaviest you can find. If the grain is perfectly straight you won't need to back your bow. Hickory should be your first choice when selecting a board
I've made bows from imperfect red oak boards and backed them with beautiful paper. They were 68 inches long, 30-40 lb at 27'' and are still alive today. No need to use dry-wall tape and make an ugly bow. I must add that if you have a board with many runnouts, their's no light backing that could save your bow on the long run.
In addition to paper and dry-wall tape, you could use rawhide (which you can order from ebay) and sinew. You can find those easily if you know hunters or if you have a butcher specialised in wild game near your home. You can alson look on ebay.
Do you have access to trees or only to boards? If you have access to trees, you won't need any backing at all.
Have fun