Help with tickets for Wimbledon Tennis

[x5x3]

I have some American colleagues coming over to visit soon and they have decided they want to go to see the tennis at WImbledon, I did try to explain that tickets are rather like gold dust but you know the Americans!

It has been a few years since I have been so not entirely sure how it works now, does anyone have any recommendations of legitimate places to buy them - I would not risk ebay/etc.

Thanks in advance.