Store brand foods are good a lot of times. They used to be garbage, but nowadays they're pretty good.
Frozen veggies instead of fresh is usually okay if you're steaming or roasting.
Automotive parts off Amazon have worked alright; Rebuilt my suspension for, like, $120. That's tie rod, sway bar, shocks, and struts. No issues for the two years since that repair.
A ton of hobbies have perfectly respectable aliexpress alternatives. Keycaps, Fountain pens, 3d printer parts. They rob intellectual property, but I like linux ISOs, so I don't exactly have a history of respecting that type of property.
Software in general can be cheaped out on; I don't think I need to champion FOSS on here.
Refrigerators and washing machines can be cheaped out on, as long as you do a bit of research about their reliability.
Lots of stuff is easy to DIY if you have some work space. Furniture, fish tanks, thermonuclear warheads. Learning to sew is valuable, not because you should make your own clothes -fuck that- but because you can mend the stitching on your current clothes.
Services can usually be cheaped out on. Youtube videos and a can-do attitude can get you through manicures and toilet repairs. Court clerks will sometimes be willing to walk you through basic legal stuff like name changes. Things you should educate yourself about beyond a short youtube video: Electricity, flammability (from heat sources), and anything involving significant pressure (pistons, compressed air, and power washers, mostly.).Also be a little careful with chemical reactions: cement hardening, for example, will produce a bit of heat. Usually this isn't a big deal and you can ignore it, but there have been idiots.The world's information is at your disposal. Provided you've got some common sense, and you never fuck around with the capacitor in a microwave, you should be fine.