Best bet is ebay, search by part number, I picked a genuine new in box R32/Anni one for mine for £14 posted. Here's all the part numbers:
Standard Mk4 mirrors...
blue, heated:
1J2857521A - mirror glass (convex) heated with carrier plate, blue tinting, left, rhd - £38.78 inc VAT
1J2857522C - mirror glass (aspherical- wide angle) heated with carrier plate, blue tinting, right, rhd - £48.36 inc VAT
clear, heated:
1J2857521E - mirror glass (convex) heated with carrier plate, left, rhd - £34.58 inc VAT
1J2857522F - mirror glass (aspherical- wide angle) heated with carrier plate, right, rhd - £42.92 inc VAT
clear, non-heated:
1J2857521 - mirror glass (convex) with carrier plate, left, rhd - £20.95 inc VAT
1J2857522 - mirror glass (aspherical- wide angle) with plate, right, rhd - £37.63 inc VAT
R32/Anniversary...
blue, heated:
1J2857521F - mirror glass (convex) heated with carrier plate, blue tinting, left, rhd - £38.78 inc VAT
1J2857522H - mirror glass (aspherical- wide angle) heated with carrier plate, blue tinting, right, rhd - £38.78 inc VAT