Sadly theres no chance of specifying.
I always ask for 'must be flat, mild stock' but rarely get it.
Your best bet is to buy extra and plead with your merchant for selecting decent stock and return it if its real carp.
In theory ivory coast stock is the best, which historically was known to be mild yellow stock, but Ive not seen that for years.
My experience is thst iroko quality is generally going downhill. The best way to select is buy generously and keep the mildest, straightest grain stock for door stiles.
If you are cutting a wide board into multiple strips, cut the board near the centre to release stress and then rip.
Some boards are unusable and best not used.
Actually, although iroko is tricky in manufacture, it is stable in service. Ive only replaced a handful of door or window sashes in the last 10 years