Just few tips like buy atleast 3 batteries which should be enought to film a whole event or even film set for a whole day. Havent had to change more than 3 times on a film set yet even while filming a whole day. Also I have used 3rd party batteries on different models and have to say I never had any problems with them and they work very well. And now found that you can get them extreme cheap from Hong Kong on ebay and are basically the same as 3rd parties that you can buy in stores but at Ebay it is way cheaper but you have to wait longer for them but its worth the wait and save your self allot of money.
To make it easy for you click here to go to Ebay search for the batteries and should directly show the batteries for the 550d or t2i, so happy shopping!