If you want something try to find it secondhand for sale or free first. So for pretty much all clothes and shoes I use Vinted and usually never pay more than £3-£10 per item on much better quality stuff than I could afford new, some of it with the tags still on. I've had amazing Whistles wool coats that would have been £400 new for £10 for example, this is great for any kids, especially teen's clothes too, give them a budget and get them searching for what they want themselves, will save you a fortune. We also go to car boot sales sometimes which can be a goldmine for even cheaper clothing (50p-£2 an item usually), this is especially good for baby items and clothes which people are often very keen to offload cheaply.
For bigger household, garden, furniture type stuff I use Facebook marketplace for most things. Set up alerts for anything you want/need and it will notify you when they get listed in the price range and distance you set. I often go and pick up items further away when we're going there anyway for something else.
For example I've been looking for a Delonghi stainless steel espresso machine and yesterday one came up on there for free and I'm collecting it tomorrow, I'm so chuffed as I was thinking I would need to spend £40-£50 even to get one secondhand.
Last weekend I picked up a sewing machine and a long handled tool for cleaning the windows too, both for free and on the way to an attraction we were visiting! The week before that I got a free Kenwood Chef, it's from the 70s but so well built and still going strong with loads of attachments, it is truly amazing what you can get for free.
I've had so many great pieces of furniture and other items for dirt cheap and free off there, it's great. I always make sure to offer anything we can't use on there too, it's so much easier than taking to a charity shop or much worse the tip.