Can you get just the top strip as a spare part for larch lap panels? I have three panels which are in reasonable nick except for the top bar which is splitting away where it's nailed on. I know I could get some stripwood and plane it to shape but it will never match the rough finish of the rest of the panels.
If you mean the slightly triangular shaped strip which stops rain from sitting on the top of the fence and gives a neat top edge it's called - Capping Rail. Comes in 6' lengths, pre - treated and costs £2.79 per length from Wickes, other suppliers available of course !
Hope that helps