GSA Bill Rates Search Tool
If you're looking to estimate what bill rate you could potentially get if you went 1099, my preferred method is to take a market salary and divide by 1000, then adjusting +/- 20% to get a range.
So $150,000 becomes $150/hour and then $120/hour - $180/hour as a low to high range.
But, you can also use this nifty tool that GSA made:
This tool basically aggregates all the bill rates for contractors that are on the GSA Multiple Award Schedule (MAS)
I just went through the process of submitting the paperwork to get on the GSA MAS and to do so, they ask for documentation that you've actually invoiced at the bill rates you are proposing, so the data you find on the site should reflect reality to some extent.
That being said, a lot of the average rates look a bit low to me so I wouldn't use this tool as your only data point.
Want the full playbook? Check out Going 1099.