If you type the words 'newtone food center' into your search engine, you'll be given websites that have those parts available.
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I did that and several other similar things even with the model number. When you just go into newtone (that is NOT the spelling) you get all different brands but not NuTone. I could not find any parts offered for model 251 except on one site they offered a motor.
From the information I find on the internet, NuTone does not manufacture the food centers any longer. I was hoping the model 272, advertised on eBay might also fit your model.
For further information on the subject, go to www.nutone.com.