They’ve got tons of different curricula (all subjects), and almost everything is offered at a discount. Free shipping for any orders over $150.

http://www.paperbackswap.com

Library and Educational Services (if you qualify for their wholesale discount…ie, if you’re HS’ing)

http://www.libraryanded.com

Homeschool Buy:

http://homeschoolbuy.com/

Plus I also recommend the previously noted Rainbow Resources and CBD.

BarnesandNoble.Com

http://www.mcgrawhill.com/

http://www.leapfrog.com/LeapFrog/index.jsp?CID=KNC-Google&HBX_PK=GeneralTerms_General+Terms_educational+resources&HBX_OU=50

Scholastic.com

Stech-vaughn.com

Beka.com

Also try eBay and your local homeschool support group.

http://www.rocksolidinc.com/

http://ebiz.netopia.com/clpress/

Visit this link, they will have multiple links, and plenty of information to help you out.

http://homeschooling.gomilpitas.com/

and

http://www.angelfire.com/or/mtdewbydo/secularcurric.html

Good luck.

they are good and reasonably priced