This is a rechargeable lithium, so you can't put a standard lithium cell in it's place.
The VL2020 is available from a number of online electronics suppliers, Google will help.
Note that you can use other sizes from the same series, e.g. the VL2030. The lithium manganese (ML series) charging regime will be compatible in this application, so you can also use the likes of the ML2020, ML2030, etc.
Be careful when you order one as there are about 6 different leg/mounting options, you need to get the one with two through hole PCB mounting leads. Else it turns in a double sided tape and bits of wire job :-)