Visit garage and yard sales. It is not uncommon to find boxes overflowing with old guides at local sales, and most people will let you take them away for pennies. Printed TV Guides were officially retired in 2005, which means a lot of people may still have their 1990s guides stashed away.
Check out thrift or secondhand book stores. These types of book stores often have expansive collections of consumer print media (magazines, almanacs) available for sale or trade. Browse around or ask a bookseller for assistance if you are looking for a particular 1990s guide.
Consult your local library. Depending on the size of your local library branch, they may have archives of historical TV Guides. Peruse through the archival stacks at your leisure or ask a librarian for assistance.
Log into your eBay account and perform a search. Using eBay is a great resource for collectors of all sorts, and the TV Guide enthusiast is no exception. If you are looking for a very specific issue of the guide, this option is your best bet.
Visit Old TV Guides at http://www.oldtvguides.com/ and browse through the site's incredible archive dating from 1948 to 2005. Every weekly issue is available and most copies are in mint condition. A great option for the serious collector.