How do you obtain the Writers Guidelines from a publisher?

Try calling them and asking...

Telephone calls are usually discouraged in the industry, though you may have some luck with it depending on the publisher. If you go on their website, they will usually have the information. Also, a quick email might work to one of their customer service reps.

Website is best, because it's most likely to be kept up-to-date. Second best would be their listing in Children's Writer's and Illustrator's Market, which any good library should have a copy of. Also, if you join SCBWI, they send out a magazine (four times a year? 6?) that lists any changes in the industry--new editors who've joined a publisher or who've been promoted, new imprints that have opened, submission policy changes, that sort of thing.

Phoning's a bad idea. Chances are you'll get a very busy (and annoyed) person who doesn't have time to read everything to you. You'll just be told to go to the website or submit through your agent.

Or worse, you'll be put through to an even busier (and VERY annoyed) editor who will tell you to go to the website or submit through your agent.

This is better:

Google the name of the publisher and "submission guidelines" and usually something will turn up.

If there are none....

Go to the 808 section of your library--or the 808 reference section--for a copy of Writers' Market 2008 and check publishers listed there. Every entry should have a section on what they're looking for and whether they accept unagented submissions.

Check the inside page of books similar to yours. The website will probably be listed there.

Penguin Group + submission guidelines

Baen Books + submission guidelines

Easy!

I agree with everyone.

I assumed (perhaps stupidly) that Jasperd was asking after having exhausted all the obvious avenues.

On websites, the submission details are often hiding somewhere on the "contacts" page.

It does seem strange, if they accept unsolicited subs, that they would make it hard for you to find out how they want things sent to them...

On the other hand, if they aren't advertising their preferences, perhaps they aren't that particular. A good query/cover letter and the first 10 pages shouldn't offend anybody!