Overstock has some that are 4-5 or 5-6. If you can't find them elsewhere, maybe you could order both sizes and see if either fits. I have returned many things to them with no problem but you have to pay postage, of course. I'd go with PP if they have them, though. With Overstock, quality varies widely in my experience.
As misskittycat mentioned, I got mine from Pearl Paradise, but they were a special order, so there are no returns allowed. I'm happy with their quality - I went for AAA and they were only about $70 in 14k gold. It took a week or two for them to come back with some options for me, but when they did, they had 3 matched pairs to choose from, and I *just* bought mine, so they may have the other 2 pairs still available if you ask.
That said, if I could return these and exchange them for the next size up, I would. If you look at my pic, you might notice a very slight airline. It IS very slight, and I went for the 5.0 - 5.5 mm size. If I had it to do over, I might have asked for 5.5 - 6.0 instead. You have to remember that the jackets are made to fit 5 mm faceted stones IN basket or martini settings, which adds a little width to the stud overall. Pearls are naturally round and taper down at the base, of course, and they don't have a metal setting around them at all. So, to get a good look, I'd really recommend at least going with the 5-5.5 that I did (wanna buy mine? ), and maybe considering upgrading to the 5.5 - 6.0 if you don't want any airline at all. (It'd be nice if I knew the exact diameter of mine - maybe I have it around here somewhere? - but if mine are closer to 5mm than 5.5 mm, then perhaps just getting a slightly larger pair within that same size group would have worked better.)