Physical books? I think Toranoana does international delivery (through Tenso) now. Melonbooks tends to have a better selection of newer stuff, so to get things from them, you need to either use a freight forwarder, or have someone you know in Japan get them for you. Websites are Japanese only, though.
Although I -do- think there are doujin middleman specialists these days.
If you want the older stories of Koharu and co, you can get them individually or a collected book (I don't know about current availability).
Comiket releases tend to become available right after Comiket proper, which means 1st Jan 2016 this time, either at 'Day 4', where Comiket doujin are sold by Melon and Tora, for a higher price (ranging from 8% (i.e. +tax) to much, much more markup), some of which does not go on to be sold later normally.
