It might be hard to find what you're after in-person unless you're in a big city with good electronics stores, and even then it's doubtful you'll find a cable specifically designed for the Slingbox.
Of course you can eBay or catalog order it, but not likely by Saturday !
I've found that Slingbox IR cables aren't particularly special, if you can find any IR Blaster-style cable it will almost certainly work. Be sure the cable is meant to transmit, not receive. Think TiVO, good computer TV tuner cards, etc. You want to hit home theater shops or consumer electronics stores with a decent A/V department. Walmart's not going to cut it. Once you find a cable the trick is making it fit in the hole.
Most non-Sling IR cables use a 3.5 mm (1/8") mono headphone plug. Slingbox uses a 2.5 mm (3/32") mono plug. If you run into this situation, you'll need to pick up a 3.5 mm > 2.5 mm mono adapter. Easiest place to find those is Radio Shack, even tiny locations should stock that part. Since you're in a hurry, I'd suggest taking the IR cable and the Slingbox with you so you can make sure everything fits.
Hope this helps, and safe travels.
Update: I didn't realize SlingMedia sold parts. Here's a link to their store, just another option to consider if there's time:
Message was edited by: ArizonaSackbut
I'm trying to controll 3 boxes with with my Slingbox Pro. The problem is the IR blaster only has 2 heads, so I'm only able to point the heads to 2 boxes!!!
Is there an easy way to do what I'm trying to do? i.e. is there a triple head IR Blaster? or how can I sacsade two dual head IR Blaster (knowing that the slingbox, I beleive only has 1 input for IR in the back)