There are all sorts of reasons a person might want to solo. Like... between groups, etc. A place I really like currently is west of Horizon. Follow the road like you're going to the ferry, and once it dips down into the lower region, around there.
There are the lower ranked variety of sphene doblyns there (200sp), mongrels(200sp), and a variety of the floating eyes (300sp). It's pretty good for solo and duo. It works well as an archer because you can pretty much 1 shot the sphenes.
A lot depends on your class, but I find that's a really nice place because it has doblyns but also gives you some other flavor. I don't suggest the sphenes to the east of the aether gate around there though because they spawn Diremites (similar to how copper bell does), so you'll have to kill them to keep sphenes spawning, and they're less optimal.
The upper tier of sphene doblyns is directly east of Uldah, so you can test them out every now and then and move over to there when you're ready. They're conveniently close to uldah, so you can grind on them while waiting for a party to get ready or whatever. Unfortunately all of these places are a bit far from r40 behests.