Shop Early - I start looking for sales in January. Seriously. When I know that I'm going to be doing shoeboxes, I keep my eyes open for deals all year long. If I have a good coupon for toothpaste and find a deal, I use the coupon and save the toothpaste for the shoeboxes.
Dollar Tree - This summer I found new Fruit of the Loom t-shirts in bright colors (perfect for boys and girls) for $1 each!! I also got fun sunglasses and hats for $1 each - these are great fillers for the shoeboxes.
Target Clearance and/or $1 Section - I scored on flip-flops for $1 at Target this summer. When Old Navy did their $1 Flip-flop sale, I headed up there for the shoeboxes, but the line was INSANE, so I went over to Target where they had cute flip-flops in their $1 section. BONUS: No line! Keep your eyes open for those lovely RED Target Clearance tags - especially when they do their 75% off Toy clearance in the fall.
Shop Back-to-School Sales - I've been able to get great deals at the end of the "Back to School" sales at Target and office supply stores on Crayola crayons, pens, pencils, pencil sharpeners, notebooks, calculators, and markers. Again, shop early in the year for the Christmas shoeboxes.
Use Real Shoeboxes - This only works if you've been buying shoes (which doesn't really save YOU any money) throughout the year. I haven't purchased any shoes in a long time, but this year I had three shoeboxes, so instead of buying plastic shoeboxes (which is what I've done in the past), I used the shoeboxes I had. I'll fill-in the rest with plastic.
I hope this helps you a bit as you pack your Shoebox for Operation Christmas Child. And remember, keep the shoeboxes in mind for NEXT year as you hit up the after-Christmas sales and throughout the year.