I was hesitant about ordering this genuine Lenovo power adapter from Amazon as some sellers were quoting prices that seem too good to be true e.g. under £30, and some as low as £10 when the genuine item costs £76 + £12 delivery direct from Lenovo.
The seller I used was ALTEC WEB.COM and for £50 with free delivery I got the genuine Lenovo Thinkpad 90w 20v power adaptor, part number 40Y7667. It came in a hologram sealed Lenovo box containing all the correct packaging, instructions and clover mains lead, exactly like you get when ordering from Lenovo.
Whilst it's not the sub £30 bargain that's on offer from some sellers, It's definitely genuine, I weighed it to make 100% sure so I know it will last because I have never had a genuine Lenovo power adapter fail on me and I have used them since Lenovo took over IBM.