Welcome to the Forum. You will find that there are people on the Forum who can answer most questions or point you in the right direction for answers. If you can answer questions as they come up, so much the better.
So to your question. Generally speaking, the plastic wheel arches are held on by two screws hidden under two plastic inserts near the top of the arch. Remove the inserts (they are glued on) by softening the glue with the heat from a hairdrier (don't use an electric paint stripper, they are far too hot). The screws will be exposed. There may be other fixings, so be careful. The covers for the screws are about £7.00 per pair which is a rip-off. If you are careful when you take them off, you may be able to reuse them. When replacing the arch, if you mask off the recesses then use a weatherproof contact adhesive on the recess and the back of the insert, you may get away with it.
With regard to replacement, the obvious source is the manufacturer if they still hold them in stock. An alternative would be to source one from a caravan breaker's yard. Unfortunately, I do not have an address so cannot help you there. If you put "caravan breakers" into the search engine on your computer, a list of caravan breakers should pop up. There will be contact numbers and so on in the list.
Another way of sorting the problem, if it is not too badly damaged, is to repair it. It is a fiddle but may be the only way out of trouble. Because you may have to match some automotive paint to the colour of the plastic of the arch, it is best to take it off the caravan.
Strengthen the repair by fixing a piece of glass fibre mat to the inside of the arch. Before the resin sets, ensure that the edges of the repair are in alignment. After the resin has set, sand down the joint with "wet and dry" paper and, if necessary, fill any holes with an automotive filler like P38. Sand down again using finer and finer paper until the repair is smooth. Give it a coat of automotive bodywork primer in the area of the repair and smooth down again with fine paper. repeat until the surface is smooth.
Clean off the arch with T-Cut to get it back to the original colour and take it to an automotive paint supplier and ask them to match it. They will then supply you with a small amount of matching paint in an aerosol can. Spray the repair taking care to go outside of the repair, rub down with a fine wet and dry paper then apply a second coat. Stand back and admire.
If you don't want to go to all that trouble, an acceptable match can be achieved by going to a Dulux paint agent (watch you don't trip over the dog) and getting them to match the paint colour. Then brush it on in the usual way. The match may not be exact, but a blind man running for his life would be glad to see it. If you paint the whole arch, the repair will not be immediately obvious.