I'm looking to buy this reed switch but I don't know how to choose a proper magnet. I would like the optimal distance to be ~1 inch.
Here's a data sheet that lists the spec of the Philips 10JK coil that is referenced in the reed switch spec, http://www.cotorelay.com/downloadables/Application%20Notes/Reed%20Switch%20Tech%20&%20App%20Section/Reed%20Switch%20Technical%20&%20Applications%20Info%20(Entire%20File).pdf
The above data sheet also gives some general info on magnet actuation of reed switched.
In most cases the newer high power magnets (Neodymium types) should have no trouble operating this reed switch. Though at a distance of 1" you may still need a fairly good size magnet, (maybe a cylindical type about an inch long by a 1/4 inch dia. - just a guess). A simple ceramic magnet may work up close but might not work at all at the distance you need.
See K&J Magnetics ( http://www.kjmagnetics.com/ ) for some ideas on small powerful magnets. Don't go crazy with a huge magnet, you may have trouble getting the switch to turn off. With a huge Neodymium magnet you'd likely have a lot of other problems too. Be sure to read the warnings that are on the K&J web site.