Rose,
How are you painting the walls? Color, etc? Any other decorations?
When my son was younger I did a baseball themed room. I found the perfect wallpaper, white with navy pinstripes. The border matched the drapes.
I'm just trying to envision how you may put the room together. If you did the same to the walls, it would blend too much, unless you intentionally made the curtains with a wider pinstripe to offset the space and/or bring in more of the blue to offset the white.
I hope I'm making sense. Typing what I'm thinking is not my forte. :lol:
Another thought, if you can't find the right fabric, and painting them on was already mentioned (just me, but I wouldn't choose that route either,) but what about white fabric and use embroidery floss and do a straight stitch to replicate the pinstripes. Another way is that it could also be done in such a way that it could look like the thickness of baseball stitching.
OK, maybe it's the lack of cold caffeine (Pepsi) affecting me right now. I getter grab a can and join the living! :lol: