A) At the time of writing (September 2009) yes, you can at Bristol as well as many other UK airports. Some airports give them out for free (London Gatwick Airport a few months ago) whereas other airports, such as Bristol and Manchester, charge £1 for either 3 or 4 bags.
Where Can You Buy Them?
Bags are usually sold from vending machines right before the final security clearance.
What Do You Need?
To buy the bags, you will need change, usually a one pound coin.
To check what needs to go INTO the bag, see this post on hand luggage restrictions.
It makes sense, therefore, to keep hold of any plastic bags you have for the next time you pass through airport security.
It also makes sense to see if you can club together with other passengers queueing up at the plastic bag dispenser as most people don’t need four plastic bags.