I don`t know if this will produce the information you are seeking but if your comparable companies are publically traded then you might be able to glean information from their SEC filings at Edgar Reports: http://www.sec.gov/edgar.shtml Plus you can always contact the company`s investor relations department to request a copy of their last annual report.
To research IPO`s try http://www.hoovers.com/global/ipoc/
When starting up, I always prefer the "ground up" method of building your projections. Attach a budget to specific marketing and sales strategies, then project the number of units sold each month.
If you scale this carefully for 12 months, it`ll give you a good idea of the cash you`ll need and give you a better chance of hitting your projections. Then, you can ramp marekting costs and market share in more general terms for subsequent years.
It`s not as sexy as the percentage of market projections, but it`s often more effective in the short term.
You can use the Google Adwords tool to find out how many people are querying the product. However, there's no way you will know how many customers are actually buying it.