OKAY...I'll help you figure this out
Rival doesn't even acknowledge that model number (3154) in their replacement parts. Makes me think we either have the wrong model # or the wrong manufacturer OR more likely we are missing a few of the prefix letters to the model number.
Please double check and make sure you are looking in the right place for it...here's a pic on Rival's website on where to look on the crockpot: http://www.crock-pot.com/CustomerService.aspx?id=rpt
If you are certain it's a Rival and the number is "3154" then call their customer support # at 1-800-323-9519.
If you discover the model number does have a few prefix letters, be sure to enter that on their search page (which is the same link I posted above.)
Not sure what the replacement part will cost but for $35 you can get a used 3154 with lid on ebay: here
(that kinda proves there is such a model - so you may have to call Rival at the number listed above.
If this solves it for ya, please let me know by rating this answer. If not let me know. :-)