If you're suffering from glare problems it's probably because the brightness and contrast settings on your monitor are far too high. I find it amazing that PC systems are supplied with all sorts of instruction books but nobody ever seems to tell you how to set up a monitor properly.
So, before you buy an anti-glare filter, take a look at this page:
http://www.poynton.com/notes/brightness_and_contrast/
(You can ignore all the technical stuff and graphs. Just scroll down to 'Monitor Adjustment'). You'll probably find that you can solve your problem (and save yourself money) by following the instructions.
If, after adjusting your monitor settings, you decide that you still need an anti-glare filter, then any decent office stationery shop should be able to supply one. You should note that they're not particularly cheap with prices starting at around �16 for a 15" monitor but rising to about �25 for a 17" monitor. (Filters are almost impossible to obtain for really large monitors; this is because such monitors aren't normally used in an office environment).
To see the range sold by Staples, take a look here:
http://www.staples.co.uk/ENG/Catalog/cat_class.asp?CatId s=1391,1395&name=UK%5FCL%5FScreen+Filters
Chris