With Doujinshi artists, if you go to Comic
Revolution, Comic Market, Super Comic City or
other doujinshi events, if you want sketches, GO
EARLY.

I handed out a bunch of Shikishi around 9-10am and
I was told to come back as early as 12 but as late
as 3pm for some people.

However, the artwork quality was well worth the
wait.  I guess the pros had a higher demand for
sketches, but the quality of the amature artists
can still be completely and utterly phenomenal.

I doubt that you can do this sort of thing in
America(unless you hit up Artists Alley.)

Keep trying, be patient and good things will come
to you.