You won't get it from a retailer for $200 when he has brick and mortar to pay for.
You have to get them from SSers who buy at auction or find other good deals and they buy several. This is the number I've seen, and used, for a couple of years now. About the same for the M105 cover. Some people will ask for $250 and get it, but $200 is an obviously better price and can be had if patient.
Keep your eyes peeled on your favorite auction site or here in the classifieds.
New ones made from retailers cost twice that. Not sure they're worth the value of the trailer that sells for that without them, but they seem to find business.
The newer covers are vinyl covered duck for better rain resistance, but the UV hardens them quickly and they will get brittle over time. If you could put a coat of wax on them once a year, even if from the car wash, it should last a little longer. Same for the rubber rope tie downs. Use some armour all wipes and keep them wiped down. Maintenance is key, and while washing helps, it is not enough by itself for long wear. Mr. Green soap and a soft brush will do wonders to restoring the old canvas to looking like new, but it won't stay there long once it turns and gets mildewed or faded, so you have to go a little further on the older ones.
