Hi drew2229,
They are available to purchase new but aren't cheap, around $180.
If you are looking for a good second hand one then I suggest you keep an eye out on Gumtree for a second hand electric 14 model 45 mower. These complete electrics sadly are worth next to nothing and if it has a good catcher it's worth buying the whole machine purely for the catcher alone, as I did. Most electrics go for between $50 and $100, no more. So that makes a pretty good buying price for a catcher and also some extra spare parts that cross over for your petrol driven 45.
Cheers,
BB