btw the links I found were through ADVrider and people who have used those services previously. all via google.
There are a few UK based outfits that rent track bikes to go with their track days. Many of them hit tracks outside of the UK, but if I remember, most go to Spain where its cheap, sunny and warm. Just Google UK bike track days, there are a few major players. Also, it's called "bike hire" in Europe, so that may help your Google efforts. And despite the standardization of the English language, i've found that most local bike/track day places still only publish in their native speak. So you may want to look up the Dutch words for track day and bike hire. Still, you'll no problem reaching out to them in English. I did a track day and rental in Spain once, that was pretty easy. I do remember researching track days and bike hires in Assen and Germany, and it was much less common. Then again, I didn't search in German or Dutch at the time.
I don't know the name of the company but there is one out of England that runs TD's at Portimao. Might be able to find it through the track's website or something.
If you are a " Facebook" user, contact this guy --- Chris van der Bij - say you know Shaun Harris and he will help you all he can.
www.crtholland.com is the link Chris just sent to me. But contact him direct also, good luck and have fun, that track is fekin awsome to ride man. The advantage to 1700 listed friends on face book who are all Bike freaks from all over the world
Be prepared for every possible weather condition. I was at the race last year and it would rain then shine every other lap.
Remember as well ... Countries in Western Europe are for the most part not big. For example, Texas and many other states would dwarf most. My point being, don't just look in the Netherlands. Look at the countries that border them as well. -jim
Okay thanks for the tips everyone. I checked with the trackday company they don't have any bikes for hire. I'll keep on searching. Worse case I'll see if there is any other trackdays near by. I just really want to ride a prestigious track like Assen.