i want to buy a wooden overtone flute, like a slovak koncovka, and i’m finding a good maker. I’ve found two sites
http://english.fujarka.cz/overtone-flute-koncovka.php
http://www.fujara.sk/instruments/folkart_slovakia/shepherd_pipes/overtone_flute.htm
the prices are pratically the same.
But i want also your opinion about overtone flutes.
I would suggest Magnar Storboekken instruments, not slovak but norwegian overtone flutes, I own two and they are both very good:
http://www.naturinstrumenter.no/ (click on “Seljefløyte / Sallow flute” link)
If you like more the slovak overtone flutes design you can find something similar in the Nadishana shop:
http://shop.nadishana.com/index.php?option=com_virtuemart&category_id=4&page=shop.browse&Itemid=1&limit=20&limitstart=0&lang=en
I have a few flutes from Marek Gonda (your first link), including koncovka. His instruments are of impeccable quality, wonderful warm sound.
Here is the picture of my koncovka:
he make very beautiful instruments, but are his overtone flutes made of bark? Aren’t they fragile? Does He produce a only wooden model? My english isn’t very good, i haven’t understood his page very well
He makes overtone flutes made out of plastic or wood.
For example the models “Polsfløyte” and “Seljefløyte lita/stor” you see in the photos on his website are made of plastic but they are covered with birch bark so they look like wood.
The all wooden models are “Heltre/Wood” and “Langfløyte” and they are more expensive.
I have a Langfløyte from Magnar Storboekken and it is a lovely overtone flute in beautiful wood with a delightful tone.
In fact, it’s so good that I don’t play it. I drag my home-made plastic Konchovka to the Folk Club and gatherings. 
You might also want to consider the glass ones: http://www.knockonwood.co.uk/details1.asp/ProductID/2908/fairy-flutes-queen.htm. I don’t have one of these, but I have considered it.
two important things: do you have the email address of Magnar Storboekken? I don’t find it on his site. What is the lenght of a Storboekken’s f or g swallow flute?
Mr Storboekken does not put his email address on his site, and for that reason I won’t pass on his email address which I ought to have somewhere. What he DOES do is have a link Beskjed/Message for you to send him a message with YOUR email address, so he can email you back. There is another similar option on the Seljefløyte (Willow Flute) page. Why don’t you ask him?
I’d guess the Willowflutes are about 600 mm long, or perhaps a bit shorter. But you are best to ask the man himself! 
The F is long about 60 cm, the D about 70 cm
As Innocent Bystander tells, the best way to contact Magnar is using the link in the “Sallow flute” page.
i’ve sent a message to mr Storboekken, thanks.
I want to ask something about Marek Gonda’s instruments, why do they cost 30-40 less than other makers? This thing generates in me some doubts.
I have three of Marek Gonda’s instruments: one koncovka, and two flutes. All of them are highest quality instruments. As far as his prices: maybe it is because he does not have middlemen to pay to. You order from him directly.