You can make your own tie-downs. Heck, you built the plane, this is easy.
I use two crossed stakes at each tie-down point. They go through an aluminum block that has a hole for the tie-down rope. The stakes are about 18" long but longer is better.
You can buy titanium rod, not cheap, but the best stake material available. McMaster sells it. Their 89145K353 is 3/8" diameter, three feet long. Cut it in half and you've got stakes for one wing - if you want longer, buy their 6' length and make three from that. You'll need to sharpen one end and put an eye or a cross-arm at the top end to give you something to pull it out with. Remember that you need to pound on the top end.
Mine are 1/2" diameter and I think that's overkill.
For ropes, please, please, don't use nylon. It's too stretchy. Use polyester or go to the Defender.com or the West Marine sites and order some rope with a hi-tech core like Dyneema, Vectran or Technora in a polyester cover. They have much less stretch than even the polyester ropes. Here's a good example:
http://tinyurl.com/4xm8r7u
This stuff has a strength of 7,800 pounds for the 5/16" size and has excellent durability.
If you're browsing you might need to search for competition marine ropes rather than recreational marine.
Double the ropes and make certain that they are tight. What I do is tie one wing tight, then the other wing, pulling it tight too. Then go to the tail (I have a taildragger) and pull the plane backwards until it creaks a bit. Then tie the tail there. Tightly.
The reason for doubling the ropes is to reduce stretch. The last thing you want is the plane moving around.
1/2" diameter ropes are fine for polyester. If you use the marine ropes I mentioned, you can drop down considerably, and I'd think that 5/16" or even 1/4" diameter would be plenty safe.
If I think there's going to be a strong wind, I will probably tie a rope from the tie-down anchors under the wings to the nearest wheels - easier to do on my Skywagon than on an RV with wheel and strut fairings.