From NUI Group Community Wiki
Currently, the most popular infrared LEDs are the Osram SFH485 or SFH485P.
Specifications: 1.5 V 100(mA) 880 (nm) 5mm. The difference between the SFH485 and SFH485P is that the first one has a beam angle of 40°. The "P" version has a wider beam angle of 80° and comes with a flat head:
Apparently, the above link no longer sells IR Leds of any kind (that I could find), so here is the same LED from Digi-Key
In doing research for the ideal LED, I came across the Osrams SFH-4350, which are identical to the 485's, but 3mm instead of 5mm, and are about 20% cheaper.
You can order them from Digi-Key also. And seeedstudio sell it starting from 10pcs: order here
NOTE: I just compared prices for the 5mm and 3mm. The cost for 100 3mm is $43.50 compared to the $38 cost of 100 5mm. It is cheaper to buy the 5mm as of this time (2/11/08) from digikey.com. -O'Malley
Another great place to buy IR LED's is on ebay. I bought them in bulk from a seller in china and they arrived very shortly.
IR emitters are an option as well. These IR emitters were linked by a commenter in a Maximum PC article about multitouch. I had success just supergluing them to the polished surface of the acrylic and then soldering wire connections between them after the glue dried. Be warned, they are pretty small and thus require some patience when working with them. The IR emitters mentioned above have the following specifications: Voltage = 1.8 V; Power = 170 mW; Current = 100 mA; Viewing Angle = 60.