Any iMac 2012 or newer have the glass and display glued together - unlike older models where the glass was held by magnets and easily replaced.
It’s now 7 years after the 2012 model came out and nobody sells just the glass, so they probably won’t due to repair difficulty and low success rate, manufacturing costs, etc.
Your only choice is to replace the whole display which is not cheap, but can be found from the usual online markets for less than what Apple would charge.