Most of the others have it right - the pre-Hogue factory grips are slimmer, and you can usually find them used or at parts catalogs like Brownell's. There are two to choose from, the standard and compact ones. You also have to get the panels and hardware. The panels are different between the two models.
About grip compatibility... GP100 "compact" grips & the SP101 grips share the same wood panels, but the grips themselves are not the same. The GP100 & Super Redhawk can use the same grips, but the standard Redhawk is completely different.
Other slim grips to choose from:
Crimson Trace makes some GP100 grips that use the same wood panels as the old factory grips, but you have to buy the panels elsewhere. I don't know if they include the hardware, either.
Eagle Grips' "Secret Service" wood grips look nice and are very trim. I don't know if they stock them or if they're a custom order.