Best warranty you will find in EU; tech products receive a mandatory 3 year warranty. Best price in EU I found in UK at eBay (304 EUR back in april) although the AC adapter is non-EU (and non-US) I expect Germany to be relatively cheap as well. A EU <-> US converter or US <-> EU converter costs like a dollar or something. Unfortunately for you, the higher price and S&H is probably not worth it.

Best price in general is in US. These prices are also usually tax-free. You might have some luck with 2nd hand N810. Be sure to ask for the recipe, know how well the screen is still (screen protector used?), and ask when and where the device was purchased. Question is how good the battery still is. In US you normally don't get extended warranty (good luck finding that). Your warranty will be 1 year however in 1 year the price will be down like 100 USD as well especially after the N810W and possibly N900 are out. If you use your N810 in a normal manner you probably won't need any warranty, and if you abuse the device this won't fall under warranty.