Where can I find the positions of the infrared band for phosphorus compounds in general? Specifically sodium hexametaphosphate
Specific for your compound you find at least two public IR spectra, either in KBr:


(source)

or from one of the larger chemical vendors as .pdf


(source, Sigma-Aldrich, product number 305553, CAS-RN 68915-31-1)

More generally speaking for phosphates, you equally find some related entries in NIST webbook chemistry here, too.

For IR-spectra in general, beside the SDBS with focus on organic compounds and sources already mentioned, there are three big multi-volume collections: Aldrich (equally one large about NMR) which is why frequently chemicals in their online catalogue have a link to either IR or/and NMR spectrum, too. The Sadtler collection is equally broad and diverse, organizes the entries by field of application (e.g., pharmaceuticals, forensics).  At least in part Sadtler is electronically available via BioRad / KnowItAll, too.  Finally Hummel, with emphazis on polymers / plastics.  Because many products of chemical synthesis are characterized by IR spectroscopy, too, you may find them indexed in databases like Reaxys (Elsevier) and Scifinder (American Chemical Society), too.