Where I can buy small size & weight transmitters (battery powered) and
a reader that would is capable to read position (3D) in regular
intervals from several transmitters and send it to computer?

Take a look on eBay, using searches like "motion capture" or "flock of birds".

Under flock of birds I have found nothing
Under motion capture there are some video devices that capture
movements. However I would like to track movements of objects using
transmitters on larger areas and in any visibility conditions.

You may want to try take a look at the following, depending on your needs:
1) Ascension Flock of Birds
http://www.ascension-tech.com/products/flockofbirds.php
2) Polhemus Fastrak
http://www.polhemus.com/fastrak.htm

flylaputa, thx. This is very close to what I am looking for. I would need  
a) more transmitters (10+)
b) longer range (up to 50 m)
...
c) lower update rate
d) lower accuracy

transmitters must be wireless

freedom of movement