You can try eBay.
Retailers of the stick are becoming limited and if you speak with staff at NOW they will most likely say that they cannot replace the remote.
Another option is to download the Roku app on your phone, which might work.
Sky are no longer selling the remotes.
Beware of the difference between what was the £4 remote at Sky, which only worked with the very early Now and Roku boxes, and the much smarter £8 remote they had for the later devices, including the Now Stick.
Most of the ‘Now’ remotes on Amazon and eBay are the earlier model, or clones of it, and the later one, if you find it, is usually so expensive that it may be cheaper to buy a new device, and get the remote that way.