My grandson is right at that "in-between" size.  I hadn't been able to find any 9 slims around our town or on many websites for that matterI was able to get him 2 pair of Levi 8-20 550 relaxed fit slip jeans and they were perfect.  The only thing better is if they had been bootcut but they were wide enough to go over his boots.  It would have also been nice to have the adjustable waist because they were just a tad loose but after a few washings I'm sure they will be fine.