Hello mate,
I found an Ipe decking board at the timber yard of my local Jewson but they only had a couple left from a previous order (not sure if they regularly stock it). I've not found anywhere that supply's hickory (except axe handles). The best timber I've bought was a lemonwood board from timberline it was 3.5x3x70 and £63 delivered. That was great bowwood and I got two bow boards out of it. I've also bought from exoti hardwoods uk and they are great, but not cheap.