Doorbell transformer is this it?
As indicated by Ecnerwal the picture you show is an old style telephone line hookup. When you go looking around for your doorbell transformer it may look like one of these:

Mounted on cover of an electrical junction box similar to the following. Liable to be found in garage or basement fairly close in distance to the bell button and bell unit. 



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Mounted to side of a ceiling light fixture junction box in garage or basement:



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Mounted on the side of the service entrance box / distribution panel.



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Mounted in some obscure place in your attic on a junction box.



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And finally it is possible that you may find a plug-in style door bell transformer that is secured to some electrical receptacle someplace. One would look similar to this.



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