Agreed with Corvidae. Also, don't underestimate the usefulness of stock assets. With some colour tweaks and an eye for cohesion, you can go very far with just stock art.
It's extremely hard to find a stranger who will be passionate for your project. In general, team passion projects arise from the developers all being friends and coming up with the game idea together, before they even decide to become a team. If you don't have friends interested in game art that are willing to develop a new idea with you, then you'll have to accept that whoever you work with will most likely not be passionate about the project, paid or not.
You said payment will make people lose passion, but that passion generally isn't there to begin with. Even working on one's own project, passionate as can be, the lack of income from it can be a passion-killer. It's very easy to get demotivated when you think about how much work you're putting in and how it's not helping with the bills that keep piling up. This is the main reason that if you want free work done for you, you have to find someone who doesn't have to worry about money, such as students living with their parents.
Another option is to find an artist who has their own game ideas, and help them make their game! This'll have the same passion problems, but the other way around - you'll be the one who'll be less passionate. But, there are a lot of artists out there who'd love to make all kinds of different projects, perhaps you'll come across one that you're really interested in. If this is something you'd like to try, then look for pixel artists making numerous similar mock-ups, and put up a post offering your free services for an artist's passion project (specify your scope limits though!). Twitter's great for this, as artists are likely to retweet such an offer even if they're not themselves interested.
