Most older Doctor Who novels are unfortunately out of print, which means you won't be able to get hold of new copies from the local bookstore. But thanks to the awesome power of the internet, and with the aid of a fair bit of cash, it should be possible to acquire any of them.
The easiest way to read a few of the books from the New Adventures and the Missing Adventures is the free ebooks available on the
BBC website. These include some of the best novels in the ranges, but I'm not sure "Human Nature", "Lungbarrow", or "the Dying Days" are ideal places to start reading, since they feature companions and concepts introduced earlier in the New Adventures.
The pseudo Missing Adventure "Who Killed Kennedy" has been re-published as an ebook by the NZDWFC, and can be read
here.
For the rest of the books, or paper copies of the ones mentioned above,
ebay is your best bet. Pretty much everything is available second hand; if nobody is selling a copy right now, there'll probably be one along in a few days. A few books tend to go for rather high prices, but many can be acquired for cover price or less.
Who One has arranged private reprints of some of the BBC EDAs, so have new copies of some titles that are hard to find elsewhere.
Spinoff books are available from
Big Finish,
Telos,
Mad Norwegian,
Random Static, and
Obverse Books.
If anyone has any tips on where to find out-of-print titles, post a comment!