I see I forgot to cover where to buy Indian States.
There are very, very few dealers I could recommend without reservation. In North America, there is Geoffrey Flack (
http://tibetanpost.com/tpost.php) and Stamps Inc. (www.stampsinc.com). In the UK, there is Gibbons (www.stanleygibbons.com/), and for George VI era in particular, Murray Payne (www.murraypayne.com).
Be wary of
ebay. There are honest sellers on
ebay, who will sell you the genuine item; there are honest sellers, who are honestly out of their depth with the Indian States, and will sell you the wrong thing, and there are outright crooks, who haven't yet been weeded out by
ebay. The same goes for all the auction houses, except Gibbons. (Gibbons is excused, because one of the directors is one of the foremost authorities on the Indian States, and heads the BPA's expert committee on Indian States as well.) Don't buy sight unseen, and don't necessarily trust the auctioneer's descriptions.
Be particularly wary of bargains. There are no bargains to be had in properly described, genuine Indian States stamps. You may get lucky and find a misdescribed genuine item. In my experience, though, you're more likely to find sellers over-identifying, rather than under-identifying, their stamps. All their geese are swans. A typical example is Jammu & Kashmir SG 1, the ½ Anna grey-black (£300 mint, £120 used): I don't know how many times I've seen SG 14, the ½ Anna deep black, (£32 mint, £200 used) offered as SG 1. (Learn how to distinguish the water-colour and oil-colour printings, and you'll save yourself a bundle - and maybe even make one.)
And try to be patient: acquire the knowledge to distinguish the genuine from the fakes. It takes time and effort, but it'll be worth it in the end. Trust me: I've been there, done that ... and I have a fine selection of fakes to show for it.