There are some great blogs and websites out there but I'm not sure about specifically free things.
Check your local CL for teacher postings. Sometimes I see posts from teachers who are exiting the profession and selling or giving away classroom things.
Scholastic has semi annual sales with really low prices. I bought a bunch of posters and bulletin borders last spring for $1 each.
I have gotten good tips by attending local association workshops and seminars. For instance, I went to the NE Assn for Science teachers and heard wonderful speakers and got several giveaways there. A local museum holds a teacher fair once a year. Many presenters and exhibitors come with lots of giveaways and ideas for resources.
Pinterest and teacherspayteachers are good sites for online ideas. A-Zreading.com and superteacherworksheets.com have great ideas but are paid membership sites - $20 a year.
Here are some places where I have found good resources. There may be areas on the websites to request free materials sometimes.
-our local art museum has a monthly program for teachers, they give out free items. They also have trunks of materials that they rent out for free for a 2 week period. I got one on Egypt last year and the kids loved it!
-US Treasury - local office also had a rent a trunk and gave away free workbooks.
-US Soybean association - I ordered free coloring books and soy crayons from them.
_zoo - they usually have items for low prices or may donate to your cause
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National Geographic for Kids, Smithsonian, Shedd Aquarium in Chicago (neat interactive coral reef) are all sites where I have either found info or received items from them.
I know some of these might be for kids older than your little ones but it's still fun to check around and see what you can find. Hopefully this will lead you to some ideas in your local area. I would give exact links but my laptop with all my info fried so I haven't yet recreated all of my favorite places.
Good luck & have fun!! There are many low cost ideas out there!