Really it is up to you to decide whether to make or buy. In making them there is the satisfaction of doing it yourself, and you can customize them anyway you want. If you just want a set to test with and see if you really like poi, but don't want to invest time and money, tennis balls in the end of long socks is painless and easy. Once you decide if you like it or not, then invest the money into a nice pair of comet or buzz poi, or make a set as per the specs on this site.
I don't know about in the Netherlands but I do know Malcolm has several nice options for poi available on this site in the shop.
As for how much time to spend, it really varies on the person. Some people catch on very quickly and there are some people who, for one reason or another, never seem to get the hang of poi. Malcolm has wonderful advice when it comes to this. Work on a single move for about 15 minutes, then switch to something else or you will get frustrated. Come back to that move later. It took me about two or so weeks at an hour or two 5 days a week to get the moves on the lessons of this site, forward and reverse. It takes longer to get things flowing and adding transitions and such. Don't look at it in terms of having to learn things in a set amount of time, just enjoy what time you do spend on spinning, and it makes the time fly by!
Hope this helps, good luck and let us know how it goes!
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Pele
Higher, higher burning fire...making music like a choir
"Oooh look! A pub!" -exclaimed after recovering from a stupid fall
"And for the decadence of art, nothing beats a roaring fire." -TMK