Most GCs will supply copper slug tape and wilkos do also they are plenty of sellers on eBay selling it too.

Also www.greengardener.co.uk

You don't need to look for the trade name - as Steve says you can get copper tape meant for the job in lots of places.
If you want to use it on the ground rather than round a pot you can fasten it to a plastic collar (perhaps staple some old flooring vinyl into a ring?) which you can then put round the plant. This is what I'd do if I ever had the heart to try dahlias again - slugs ate everything right down to ground level last time I tried.