I will first ask for things I want/need on Freecycle. If I come up dry I'll check thrift stores for things. My kids aren't spoiled brats (not saying yours are! Just, your 15 yr old is sounding like one atm, and they all can at moments) so they know that their things won't be new, and they agree it's even better when it's cheaper. They like to both get gifts AND eat, rather than having to choose, lol. If I get something pretty good at a thrift store but it needs some work I can do it, or pay someone less than it would cost to buy the thing new. I also keep my eye out at yard sales year round, flea market-type sales, mom-to-mom sales, etc. I found a vanity marked $7 on half off day, so cost me $3.50, at the Volunteers of America store! Had a loose leg and some scratches. I tightened it up and touched up the scratches. DD1 now has a great gift. Got some roller skates for ds last year and took them to a repair place for new wheels. Looked brand new.
For better/higher priced things, do you have any skills you could barter? There's a barter section on CL, for instance. I can build a website, or sew. Dh was a professional painter before the economy tanked, so he'll barter painting for all kinds of things, like half a cow (beef) or various other things. Even the kids are in 4H and raise rabbits (in 2 hutches on the back patio of our townhome. Very easy animal to raise in a tight space). They sell or trade pedigreed bunnies for money for gifts or things they want/"need". At $20-30/bunny, 6 in a litter, able to rebreed as often as once a month (we only breed a rabbit once to twice per year) they do all right!
Takes some degree of ability to ignore rejection, but you can just cold email people selling things on CL, too, asking them to barter for something. Maybe you have an older PS with 100 games and they'd take that in exchange for an Xbox 360. I bought a new computer this year (w/student loans, for school) and it came with an Xbox. They're kind of a dime a dozen right now, so you may be able to put up a "wanted, let me know what I can do for you in return" ad in the electronics section of CL (well, maybe not those words, since that sounds kind of whorey, lol). And don't forget pawn shops. They've got really nice things at 75-50% of the cost of new.