If you use say a TSAL6100, I doubt you would need a collimator for your application. The TSAL6100 is a narrow beam IR LED wavelength matched ( 940nm) to Vishay's and Panasonics IR detectors. I've used Panasonics PNA4602M and the TSAL6100 with no optics to 50ft but I've read of people who've far surpassed this. The tsal6100 has a high radiant intensity 1000 mW/sr at 1 amp pulsed current even considering
30% losses in side lobes for the price they are about the best bang for the buck ,particularly when you consider you wouldn't need optics.TSAL6100 Data sheet
PNA4602M
Appnote on the basic's selecting suitable LED's
If you find you do need to build a collimator
You can use surplus optics and mirrors a list of dealers is here. Bottom of the page.
Typically you would use a 40 to 60 degree emitter when using an optical system not a narrow one like described on the above page.