I would avoid eBay leather. You could get lucky but I wouldn't risk the waste of time and energy if the leather is crap. I usually don't recommend Tandy but they may have something to fit your need for a bit more certainty than eBay.
Any 6 to 10 oz veg/oil tan should work for that back piece. I wouldn't go much lower than 6 and anything higher than 10 is just going to add bulk and weight.
Also, aesthetically, you get nothing from dye other than color so if you'd be inclined to use plain veg tan you could let it develop it's own patina/character with time.
Best of luck on the resto.