Where can I find scientific poster examples?
See Colin Purrington's advice on designing conference posters.  You can find it here: http://colinpurrington.com/tips/poster-design

For more examples than you'll ever want, see the comment above by Danny Ruijters.

(I know we frown on link-only answers here, but Purrington's pages have been very stable for a very long time, and the advice is too lengthy even to try to summarize here.)