Is there something wrong with your existing BB? Chances are you can remove it, clean the bearings and races, re-grease and be good to go. If it's toast then most likely you can go with a Shimano UN-54 for about $20. Probably a 110mm or 113mm spindle if you have a double crank. I buy it locally so if the spindle length is not just right I can exchange it for the correct length.

<example> http://www.jensonusa.com/store/produ...m+Bracket.aspx