Your containers don't need to be "bento" containers - anything with a good seal will do! And even if it doesn't have a good seal, you can use a large rubber band or hair elastic around it to keep it closed, a lot of authentic bento boxes use elastic straps for that purpose. It doesn't need to have sections either, you can make your own dividers.
Rubber band bento strap, foil muffin cup divider
And look in your fridge - I've seen several brands of lunch meats being sold in reusable plastic containers that would work perfectly for bento lunches. The containers are sturdy, seal well, and are microwaveable and top rack dishwasher safe. I reuse the ones our lunch meats come in for Sailor Boy's lunches. If the containers don't make it home, I'm not out anything. If you're going to be buying lunch meat anyways, why not reuse the containers rather than just recycle them!
Deli meat containers = bento boxes
Divided sections are handy, but not necessary. In fact, not having sections gives you a lot of flexibility :) You may already have things you could use for dividers - silicone baking cups, take out condiment containers, little baby food dishes ... you can even use some foods as dividers between 2 others - string cheese, a wall of crackers, melba toast ... you get the picture.
String cheese & paper muffin cups as dividersYour kids will love their bento lunches even without embellishments. But they ARE fun to do, especially if you have a theme for the day. You don't need to buy special stamps or cutters though - look in the baking drawer, and your kids' art supplies! Cookie cutters, Jell-o Jigglers molds, Play-doh stamps and accessories, swizzle stick skewers... you could even try cutting out little paper embellishments ... there are a lot of ways to decorate your bentos without spending any money - use your imagination!
Play-doh cutters and mini Play-doh tub = cheese shapes and dip cup
Ice cube trays = rice molds
One thing I've noticed about bento, is there doesn't really seem to be a strict set of rules to follow... so you're free to think outside the box!