Those are not purchased.  They are made with gum paste.  You can make them by buying square cake dummies 1" - 1.5" larger than the cakes will be and a roll out gum paste and dry it on the dummies with flared out sides.  Make them well in advance so they will have time to fully dry.  And don't make them too thin or they may break.   I would make 1 extra just in cause.  After your cakes are covered in fondant gently put them into the  "paper liners".

Hope that helps.  

Could this also work with pastillage?