Clare, I really don't know, but here's a thread from February discussing the same question. It sounds like it may be possible to order from Harbor Freight, but I wonder if the cost is prohibitive. You might call the toll free number to find out cost with shipping. I hope it's workable!
HFGH replacement panels

Sorry to hear about the 90 mph winds...I hope you didn't lose any plants.

Mudhouse - thanks for the link. I will give them a call and find out cost. I am glad that is all I lost compared to other people in the area.
Clare

Poly panels can be repaired using a solvent,( metheline chloride? plastic stores sell it).

Save the broken panels for future repairs.

4mm poly panels can be cut very easily. May be cheaper to buy a whole 4' x 8' panel.

dcarch

dcarch - Thanks for the information.

I had saved the plastic from when we cut out holes in the panels for the fan and vent, so hopefully can fix the broken corner. Another panel blew off last night but we found it at almost the end of the 2.5 acres. Must have had another big gust. The hard part was find all the clips. It was like an Easter egg hunt.
Clare

Hi Clare - I too live in high wind area of Riverside County. Ever since we use the self taping screw to hold in the panels, I have no problem.

Kasie

Kasie - Thanks for the tip, but it was hard enough to get DH to put it together in the first place so not going to ask him to do any more on it then has to. I wish we would have used screws or sealed it in with caulk or anything else so the panels wouldn't blow away.
Clare