I expect that this part is discontinued, so you might try ebay but then would want to press in new bushings, same as one from a junkyard.
Aftermarket parts support only has the BJ piece you still have, or what most people would do in tis situation i sthat instead aftermarket for the missing piece uses a one piece control arm to replace both of the two pieces. It's a mirror image of the left side upper control arm.
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It removes the ease in adjusting castor that the two piece allows, but the castor is still adjustable to match the left side, using an eccentric bolt/plate kit.
Suspension Parts and More for Your 1997 FORD EXPLORER at Reliably Low Prices. Fast Online Catalog. DIY-Easy. Your Choice of Quality. Full Manufacturer Warranty.
www.rockauto.com
The downside to replacing the two piece with the one piece, is that with the two piece if you record how far out the adjustment bolt was, you might get close enough to adjusting the new one to the correct castor to not need an alignment done. If you instead use the one piece and the bolt/plate kit, odds are the alignment will be much further off and need realigned. Then again, if doing the left side, and maybe the tie rod ends at the same time is a good idea too, then can just get one alignment done after replacing all that.