These items are available at a lot of different places. Carter & Holmes
(online) has min/max thermometers ($27), but so does Home Depot, which
is where I got mine. Most online orchid supply places have hygrometers,
but my experience has been that they vary in accuracy. My most accurate
hygrometer is a cheap one that I got at a local nursery. BTW,
Charley's Greenhouse is a good Internet source for orchid tools.

Another tool that you might consider is a meter that measures light
from various sources--daylight, HID, and flourescent. It was a real
eye-opener for me--I didn't have nearly as much light as I thought I
had.

If you belong ot an orchid society, they may have some tools on loan to
measure light, water quality, etc., or perhaps they can recommend local
sources for the tools you are seeking.