There's a slipcover made for a sectional couch here
Or you may simply want to do a search for slipcover at http://www.voila.ecrater.com/
Hope it helps you!
Have you tried fitted bed sheets? They have elastic and can be tucked in. Not sure if they will help, but it's worth a try. I use fitted sheets as they are easily removed and are machine washable. Use a the size of fitted sheet, best suited to your couch size. Good luck.
Overstock.com has a very good selection of stretch slipcovers like the one I bought. They never charge more than $3 shipping and it is easy to return items.
Do you have a store called Tuesday Morning near you? 2 days ago, I got a Sure Fit slipcover for a sofa there for $60 that cost $129 plus tax and shipping on the SureFit website in their current catalog. They had a discounted chair cover I got for $24. Ross Stores often have discounted slipcovers too.
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