First of all I applaud you for deciding to move forward with a VA Service Provider instead of hiring a VA directly because worst case scenario if the VA is not able to work or leaves then your work is on standstill. That said when you go with a VA Service Provider you are guaranteed continuous work and replacement of your VA if you need be because providing clients VAs is their entire business model.
In regards to finding the best sites/resources for outsourcing admin tasks and so on, you can either go the Do It Yourself (DIY) or the Done For You (DFY) route.
Going the DIY route:
1) Check out Virtual Assistant Forums such as VirtualAssistantForums.com and VaNetworking.com. Virtual Assistants from around the world hang out on sites like this to network and exchange ideas. You can go on there post your job request.
2) Check out Freelancing sites such as Odesk.com, Elance.com and the great thing about sites likes this is that you can get to review the feedback left for the providers and ratings on similar work that they have completed.
3) Make use of Job Boards like and if you are specifically looking to place ads for Philippine based Virtual Assistants, check out Jobstreet.com.ph and BestJobs.ph. Just be ready for a flood of responses.
Going the DFY route:
1) Virtual Assistant Placement companies such as Zirtual.com and VirtualStaffFinder.com for a one time fee will help you find a VA who matches your requirements and can get the job done.
2) Virtual Assistant Services Providers are firms like ContemporaryVA.com. AskSunday.com and HireYourVirtualAssistant.com (full disclosure - this is my firm) who provide entrepreneurs with a team of Virtual Assistants and also project managers to handle all your time consuming business tasks so that you can focus on what really matters in your business.
My final point is that even though you find a Virtual Assistant to help implement the work, you have to realize that you have to monitor and manage the process so that you get the outcome you want. It's never going to be totally hands off don't believe the hype! That said when you hire the right VA, or team of VAs they will make your experience pleasant!