I got mine from a plastic repair specialist in New Zealand ( Techspan RPCPET3RGI- PC/PET- JD Green- 3mm round- 25 lengths )
In the US Pollyvance is apparently the go to
Apparently their welding kit is quite good as it has all of the tools needed
Polyvance welding kit
or just the rods
Green Pollycarbonate welding rods
JD use Xenoy - Valox blend which is a variation on std pollycarbonate supposed to be more UV resistant
Be careful.
I have seen LDPE rods listed as JD Green .
Most rainwater tanks are LDPE so most of the green welding rods advertised are LDPE.
If they are cheap they are LDPE and it won't take to Xenoy