I used plain blue curtains from Curtain Wonderland for my DS's room. I got sheer ones as he has blinds already & they were more for decoration. From memory they were about $20 on sale. They were floor length.

For DD's room, I used fabric from IKEA & made some tab tops using a guide I found via google and adjusted it a bit. I loved IKEA's range of fabrics. They had heaps that would be suitable for boys or girls & fairly cheap. DD's curtains cost me about $50 by the time I bought material, buttons & thread. The material was about $10 per metre.

If you can sew, & really like the ones that are too short, you could sew horizontal panels into them in a plain block colour that either matches or contrasts the print.