Availiable from most tackle shops as a farmed bait or dug. Alternatively you can dig your own if you know where.

There are several species but for ease i'll generalise and split them into 3 groups by average size:

Maddies

Small harbour rag are found in dense colonys in soft mud, they're a delicate worm of 2-3inches.
They are generally best head hooked in bunches to create a lively wriggleing mass, good for flatties, bass and eels, just about anything really.
I'm told mullet will take them but i'm yet to catch a mullet by this method.

Red/Harbour

Larger and tougher than maddies at about 3-6 inches found in inter tidal mud/sand/ shingle mixtures. Use whole, in bunches or sections on small hooks for small species.

King Ragworm

Larger still, up to 18 inches maybe bigger, but mostly 6-12. tend to be a definate green in colour, found as above.

White Rag

Some times known as snake whites for the larger ones, these beastys are a bit different. Sizes, up to about 6 inches and as you might have gathered, they're white. Unlike the ones previously mentioned they are usually found in sand when digging lug or razors etc.near the low tide mark. They are best kept in a tray or bucket of shallow water. I've never seen bait shops stock them as they are difficult to keep. Match anglers luv 'em, they're very lively when fresh and make an attractive sight bait in clear water, usually used as a tip bait ,head hooked & left to wriggle.

Maddies usually have to be dug as they seem to be rarely availiable from shops due to being delicate and difficult to keep, red rag or kings can usually be bought from tackle shops, either dug from the shore or farmed either wrapped in mikalite, or in sea peat.

As a bait they work best when live as both a sight & scent bait, they are a lot less effective as a dead bait though I've had a little success with salting and freezing large kings and using them after dark. Once I've dug some I sort through my bucket & discard any broken or dammaged worms unless I'm gonna use them straight away. Cleaning any mud & draining the water off to toughen them up a bit by drying them out I put some newspaper in the bucket to soak up excess water (mikalite does the same job).

Exposure to heat will kill them quickly and turn them to a nasty stinking mush. Keep them in a cool place short term, the fridge if the wife will let ya (the fresh water condensation will kill them off here if left too long or you don't change the paper wraps). If you've got a bait fridge ( or a very understanding wife) keep them in the fridge in a very shallow tray of sea water. Even better, if you've a dedicated bait fridge with a small water pump so you can circulate sea water through the tray.