As already announced, the Lumia 950 XL is pre-orderable (is that a word?) for £529.99 and the Lumia 950 for £449.99 (here in the UK).
WC reports that pre-orders are also now active in Italy too (the Lumia 950 for €599 and the Lumia 950 XL for €699) and I'd bet on other European country sites getting some action too over the next week, so keep your eyes peeled.
The wait between announcement and availability seems especially long this time - partly because of the absence of true flagships from the Windows line-up for 18 months, and partly because the industry has moved on in terms of logistics, now in late 2015. We now expect a company to announce a device and have it available within a matter of 'weeks', at most - the two month delay in Microsoft's case is... unfortunate, and reminiscent of the delays for a new device five years ago under Nokia. The excuse is a good one - that Windows 10 Mobile isn't actually finished yet. But it doesn't make the wait any easier!