You can but frozen mussels in kilo bags from Tescos. Just defrost as many as you need and use in a mix, on a hair or on a big hook.
I have caught some big carp and the odd tench on them, but they are fragile as a hookbait, so I use them close in where a gentle lob cast is sufficient.
I prefer cockles to be honest - they are much more robust and you can hair rig them in multiples to make the hookbait as big or small as you want.