Hi Susytwo:
I am by no means an expert, but have made numerous fondant covered cake over the years.
You don't need a super thick layer of buttercream on the outside, but you want enough that you can cover the cake smoothly, as the fondant will show lumps and bumps and dips that are underneath it. Whatever you do, do not used canned frosting - it will not hold up and could well be the ruin of your cake. Not sure what your time constraints are for making more buttercream, but you could use ganache to cover and or fill the cake layers or some of them. It's very quick to make, just heat some heavy cream and pour over chopped up chocolate. (google for exact proportions - I am not at home right now.)I actually prefer to cover the cake with ganache before fondant- I always get better results than I do with buttercream.
You can use other fillings like lemon curd, preserves, mousse, but it is best to have a generous dam of buttercream on each layer to hold those fillings in. I think you will have a much easier time of things if you can manage to make more buttercream - I always make twice as much as I think I need and often end up using a good deal of the extra.
Have you already torted your cake (cut it into layers?) If not, maybe you could make just three layers instead of five. An advantage to this is that not only will you need less buttercream, but the fewer layers you have, the more stable your cake is.
Jane