This will by no means get you a complete list, but it's a good starting point. Go to http://catalog.loc.gov/ and choose "Basic Search", then choose "Subject Browse" as a search type. Type in "alternative histories" (this is a genre heading rather than a true subject heading, which means it will seek out all fiction coded to this genre). You'll get two hits right at the top of the results list (looks like the genre heading has been officially altered, but Library of Congress hasn't caught up on the housecleaning of old records yet, tsk tsk) ... there are 123 title hits under the first and 109 under the second. These will give you a good selection of authors who write this type of fiction. To narrow down further, go into the full record for any selected title and the subject headings listed will give you an idea of which era/time split the book is concentrating on. Hopefully you'll be able to find some of the titles that interest you in your local library (don't forget to politely hassle the staff for interlibrary loans of titles not locally available ... all public libraries ARE supposed to offer this service).
Happy hunting and reading!