Where to buy a large picnic blanket?

I need a large picnic blanket suitable for 10+ people to sit upon during tennis matches. It doesn't matter if the shape is square or highly rectangular. Since this is going to be placed upon wet grass it is a necessity that it has the "water resistant bottom". I would prefer that both sides were made of the water-resistant fabric, but after doing some research it doesn't seem plausible.

I want something a bit softer than just a tarp, the ground can be very uncomfortable. Target used to sell some good ones, but they were small and aren't sold there anymore.

I can always buy two and sew them together. However, you should assume that I am a very dismal sewer and would be incapable of making my own blanket. My budget is $40.

As for colors, I would prefer red, yellow, black, or grey since these are our school colors, but I suppose I can always cover it with something prettier. Price is the number one priority.

Thank you for your time.