I cannot remember if they are Surefit but I have some slipcovers I got from Target and they have been wonderful. I have one on a three cushion sofa and one on a loveseat.
My MIL and SIL taught me a trick (I had never heard of this before) - save paper towel/and or toilet paper tubes and tuck them in the corners and along the back - no slipping! Works great!
I just bought a loveseat 2 piece stretch Surefit new on ebay and I'm thrilled with it. It's amazing. The kids jump all over it and it stays perfect. I think the key is to buy this type unless you want to be straigthening it up constantly. Mine is taupe microfiber and I paid $60 with shipping.