Bulkier than IMes is the NHC ligand IPr (CAS 244187-81-3).  IPr features diisopropylphenyl in place of the mesityl substituents.  
Some variants of IMes and IPr have saturated backbones, two such ligands are SIMes and SIPr.  They are prepared by alkylation of substituted anilines with dibromoethane followed by ring closure and dehydrohalogenation of the dihydroimidazolium salt.