It sort of depends on the size and material. You can use any kind of sink you like and buy them in any plumbing supply store or online. However, some RVs use sizes or light weight materials (plastic) that are not typical for residential or mobile home use and are more easily found in RV stores (but at a higher price). Regular plumbing and home supply stores can usually order most any size or type you want, though.
For a galley sink I would probably choose stainless steel, but maybe your decor would make something else a better choice.
Check with a Lowes or Home Depot first, but here are a couple online sources...
http://www.pplmotorhomes.com/parts/rv-pumps-water-filters-fixtures-1.htm#Sinks%20-%20RV
http://mobilehomepartsstore.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=rv-sinks-and-drains