Genoveva, I've had my Virtual Assistant business for 2.5 years and only worked as a sub-contractor for someone else for a short time. That position came from networking with other VAs and letting them get to know and trust me.
I have seen some multi-VA businesses that post for VAs, but I haven't paid much attention as they tend to pay their subcontractors a lot lower than the industry standard. (The industry standard for VAs is $20+/hr depending on the specialty and sub-contractors typically are paid 70 - 80% of that.)
If it's something you are serious about I'd take a good look at how many hours you can work, how much you need to earn, what your skills are go from there. Can you afford to make $8-$10/hr or do you need (or want) more than that? VAnetworking is a great place to learn about the industry, get to know other VAs, and I frequently see RFPs come through for subcontractor positions from their VAinsider program.
VAinsiders
I have a free report on my blog for how to set your rates, if you want to take a look at the breakdown of your pricing-to-income goals.
The Work at Home Wife
Best of luck!