My toilet in constantly filling up and hissing
If your toilet doesn't stop filling up or is constantly hissing it is down to two issues, the first would be the bottom seal on the flushing valve being warped in some way, constantly letting water flow into the pan, and such having the inlet valve fill up the cistern constantly.
Or the more second common issue, with the internal diaphragm.
Unfixed inlet valves can add a hefty amount to your water bill if left unattended, and should hopefully be fixed at the nearest possible time.
We stock a large range of the UK's leading brands, if you can identify the brand / make, you can more than likely source the seal or diaphragm that you require.