There are lots of results for retailers if you do a search for the part number C&G in FL gave above.
Reading threads on the forum for the replacement technique is also useful.
I didn't like that my escape hatch yellowed so quickly so, like others here, when I replaced it I also painted it with white Krylon plastics paint. The paint still allows light to come through albeit maybe not quite as much.
To help mitigate the effects of UV on all plastic parts I began using another product recommended by some ppl on the forum:
303 Aerospace Protectant. It needs to be reapplied periodically.