If you have the time, check out the khymos website. It's main attraction is a freely downloadable collection of recipes that use all sorts of different gelling agents. There's a little bit of information on each one, such as suggested ratios etc etc. Considering it's free, there's no reason not to have a look at it. At the very least, you can consider it a definitive list of every available option you have.
While agar is fairly easy to get, you'll probably get better results with either carageenans or gellan. I find agar gels are too firm for a pleasant dessert, although that's personal. Many of the dairy desserts you buy in supermarkets use carageenans (which come from seaweed), and it's easy to mail-order.
There are two types of gellan that give different results and you can mix them to get the exact texture you're after. However gellan is more expensive and I can understand that you might not have the time to experiment.
I'll also add that the most common mousse I make is simply chocolate, and it doesn't have any gelatine in it at all. It sets with just egg yolks (I use a recipe that cooks the egg yolks so there's no risk of salmonella).