I wouldnt get a header for the '90 since its got the RENIX head (unless you have swapped heads).
I got this one from ebay...
http://cgi.ebay.com/ebaymotors/JEEP-CHEROKEE-COMANCHE-4-0-87-90-EXHAUST-MANIFOLD-NEW_W0QQitemZ310002048172QQihZ021QQcategoryZ33632QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
It hasnt arrived yet. This seller shipped it right away, but it will end up taking a week and two days to get here (I will get it friday ordered it last wednesday). Im ordering it to replace my OEM.
If you want to get a header, then think about swapping to the HO (91-97) head and convert the top end. (Its just basically swapping parts) Then get a header for that.
Our RENIX systems (i have a 90 also) dont flow that well, and getting a header will end up being a waste of money. This is my opinion of course, and i believe i have also read this a few times i just cant remember where.
I have plans to swap in a stroker with the HO head etc... so that will be my future. If you dont plan on rebuilding an engine for yours, then deeply consider swapping at least everything from the head up in from a HO jeep.
If you dont want to do either of those things, then it would be my recomendation to just get a generic OEM replacement Exhaust Manifold for the renix system.
Hope that helps.