Closed sales data from public sites is always very out of date. It is also often unreliable. You are much better off having your buyers agent do the CMA for you as part of representing you in the transaction. The best you can get from a public site is a price and a date, and it will be dated (3 months at best, usually much older). An agent will be able to access sales that closed today and last week. That information is simply not available on any site. But it is available to your agent. Also, as part of the CMA, we examine each sale by interviewing the agent or even the buyer and seeing if there were concessions or indications that the sale was not arms length. Finally, besides the sale price and the date, we need to know the condition, size, and features of the property that sold to compare it with the home that you attempting to buy. There are also a lot of intangibles to evaluate, such as location, appeal, view, etc.
This is really beyond the scope of the typical buyer and needs to be done by an experienced professional. If you have any further questions or would like additional tips, contact me through my website. Good luck with your purchase!
While most REALTOR mls sites are not open for public viewing, REALTORS also use the information found on the state or local tax rolls. I'm not familiar with your area or data sources, so I would suggest you Google up your county tax department and find out if there is public access. If you can find the tax parcel or ID number of one of the condos in the development you are interested in, you can take the front numbers (eliminate the last number which is usually the parcel or lot #) and do a global search of the subdivision or development.
I am sorry to say that there really are not any publicly available sites that will offer information accurate enough to make this a DIY kind of project. The most important info will be about recent sales. The MLS is best place to get accurate sold data; including days on the market, square footage, and any concessions that may have been paid by the seller.
Shop a few brokers until you find one that is willing to offer you the services and information you need at a price you can afford, or hire yourself a full service Buyer's agent and let the seller pay your agent after he has helped you negotiate and close the deal.
While you can pull data from public sites, I agree with Jean that you can get this from your buyer agent. Your buyer agent may omit certain properties that you might include if you were to do it on your own, or visa versa. Transactions need to be arms length in order to be relevant comps. Concessions or type of financing may impact a sales price and must be factored when looking at comps.
Some buyers or sellers will try to do their own comps by pulling data from sites like Truia, Zillow, or Realtor.com. I encourage consumers to be informed and active participants. In the end, I do belive that most consumers are better served by having the guidance of a agent representing them.
For sources, try the sites mentioned above.