I need some good followers for a thief-style character, especially followers who are adept in sneaking. I've already used Faendal, but he got really weak after about level 10. So, any suggestions and their locations?

Have you completed the companions?

^No I haven't

Derkeethus, Anneke Crag-Jumper, and Jenassa have adequate sneak skill, and are relatively good when their boots have been muffled.

Derkeethus and Anneke both live in Darkwater Crossing. Derkeethus must be rescued from Darkwater Pass first, though. As a bonus, thanks to a bug, he's essential.

Jenassa can be hired in Whiterun.

There's also Brelyna Maryon and Marcurio, both mages, though with high sneak. Marcurio's a hireling, Brelyna's at the College of Winterhold.

Mercer Frey can be stolen from the quest where he follows you.

Cicero and the Dark Brotherhood initiates are the followers with the highest sneak skill. And I suppose Aela has a high sneak skill, but the Companions are a little outside you range, eh, sneak-thief?

If you like console commands, Veezara and Gulum-Ei would both have all the proper dialogue for following if added to the faction, and their dispositions improved. Obviously there are the Thieves Guild members, but I know a few have unique voices and thus couldn't follow.

What about the Dark Brotherhood? If not, then this person is probably your best shot http://uesp.net/wiki/Skyrim:Jenassa

1. Cicero, sneak 100
2. Aela, sneak 97 <---------
3. Eola, sneak 78
4. Marcurio, sneak 73
5. Faendal sneak 66
Jenassa 49

Anyways, follower stats are currently not updated with level. Dismiss him, then disable and enable. Faendal's stats will be updated to current level, and he will be much better.

Thanks for the info. I just started the companion's questline to get Aela as a follower because I prefer someone who's extremely adept with a bow as well as sneaking.

Hmmm ... but to get Aela, you'll have to
Not sure if that's something you've planned for your character ...

Being a werewolf is fine by me. I was planning to turn into one anyways to be like Professor Lupin from the Harry Potter series lol