I'd imagine you could get a replacement in a spare parts shop. There is one on North Frederick St in Dublin (less than 5 min walk from O' Connell St). You'll probably need to quote the make and model of the microwave.
A great place for spare parts is your local recycleing depot.
I have got parts for appliances out of machines droped in for recycling .( just slip the operator a few bob for a pint)
Are turntables actually required in microwave?
I got a new micorwave and there's no turntable.
It also includes a metal shelf so presumably the old rule of not putting metal in the microwave is also false.