You’ve tagged this in the NOW streaming stick/boxes which is nothing to do with the router. Best to call the broadband team using the number from the link below.
Is this the mains adapter with the weird blue plug on the end of the wire?
Get a replacement router with power supply included off eBay, like this:-
for probably less than a new power supply would cost you, even if you could find one.
There doesn't seem to be anything special about the Now power supply, most ISP supplied routers seem to use an equivalent one.
So if you (or someone you know) have an old router the PSU will probably work.
Just confirm (it will be printed on the plug) That it is 12V, centre pin +ve, and at least 2A. (slightly higher. like 2.5A wont hurt, it just means it wont need to work as hard as it could)
The NOW Hub router requires a 12V power supply with a minimum rating of 2A, the connector on the other end appears to be a 4.8mm x 1.7mm male barrel connector plug. Although barrel connectors all look similar there are lots of subtly different standard sizes of both outside diameter (the first number) and pin diameter (the second number) such that finding a suitable old power supply laying around would be relatively unlikely.
Universal power supplies that meet the above criteria are readily available (often coming with a selection of barrel plug sizes). I however like @RoyB's suggestion of buying a used NOW Hub with power supply included as most universal power supplies sold on online marketplaces are unbranded Chinese imports of possibly questionable provenance, buying a used genuine NOW power supply would give you piece of mind that it's safe.