Hi Brian!
Actually there are a number of Panel services on the web today. One I am personally familiar with is SSI (Survey Sampling International) you can contact them through their website at: http://www.surveysampling.com/
There are others too. I believe Harris Interactive has panels, and I know of two others that have been used with SG (but their names escape me at the moment — if I remember them I will post them here)
Also, as an alternative: Depending on the particular research you are going your might be able to trade data points with other online organizations to ‘piggy back’ on their newsletters. It can mean a bit more legwork, but can also save you a lot of money.
If you’d like some more advice just let me know. I hope this helps!
Cheers,
-Christian