Trying to find something with that short a notice means you're going to need to call around to stores to see if they have exactly what you're looking for.
Calvin's shirt is actually a red shirt with thin black stripes, but if you found something in store that is close, as long as you had a stuffed tiger and the blonde hair, you should look good.
Old Navy's website has teeshirts that would work, but what you need to do is call some nearby stores and ask them if they have the shirt in stock in your size (and give them the item number to look up)
defaults to the blue shirt, but they do have a red version - make sure that the tell the store you call you're looking for the red one!
If you can't find a thin striped red tee shirt, then find a solid red tee shirt, a super thick black sharpie, duct tape, and a cardboard box. Flatten the box and stretch the shirt over the box so it is nice and flat and no wrinkles (may have to cut down the cardboard slightly) basically, you want the tee shirt flat and wrinkle free as you are going to draw the stripes on the shirt. Cut down some cardboard to do the same to the sleeve areas too.
Take the tape, and use strips of it across the width of the tee shirt as a guide for the sharpie stripe. Then use the duct tape as a guide to draw in the stripe as straight as possible with the sharpie marker.
As far as finding the stuffed tiger, oh man... Bill Watterson would not allow any usage of his characters, so there are no Hobbes the Tigers out there, and finding one that looks similar is unlikely. I'd be happy just to find a stuffed tiger, especially on short notice. Again, I'd google toy stores in your area and start calling to see if they have one and ask them to hold it for you and run RIGHT over.