Alkaline and rechargeable NiMH batteries. The price per hours of use favors the cheap brands. A top brand might last longer but you are paying a premium for it. Rechargeable NiMH batteries with a good price to battery life ratio are from IKEA.

Toilet paper, trash bags, paper towels. If you go the absolute cheapest, they're arguably defective, but the second cheapest is usually ok.

Saline nasal spray. Just get the generic. It's just freaking salt water.

Cloths, headphones, handbags. Anything that's just buying the brand.

I don't get why I should want these high end brands when the only thing I can afford from them is plastered with their logo.

I've seen the Gucci tracksuit, the Jordans, the Beats, everything and I'm not impressed. Even though luxury things are luxury I don't even agree with the luxuriousness of many of these products. For example, if I'm going to wear a Gucci tracksuit covered in the Gucci logo and using Gucci colors then it's gotta look good first and foremost. If I'm going to hold a Hermes bag that's not comfortable, or durable, or robust but is just supposed to look cool, then it's gotta actually look cool! Not like something Shien could design.

And yes I am aware of the concept of buying a store of value: diamonds, expensive watches, actually rare and valued handbags. But most of the famous luxuries I see in public are not that. They're literally a poor man's status symbol IMO.

You should just stop spending money in general if you can. So many responses are like "buy literal garbage like disposable decorations at the dollar store," do not do that. Stop spending money on garbage. Every dollar you spend is damage to the environment and more power for the wealthy, just stop it. Stop spending.
Don't buy entertainment, use archive dot org. Read more. When you like something culture you consumed, give it to somebody else so they don't try to buy it. Pass entertainment around to your friends.
When you have to spend on things like haircuts, repairs, etc, keep the money in your social network. If you don't cook, learn. If you aren't a part of your local grocery distro, join it.

Avoid giving corpos money whenever you can and you'll save lots of money.

Dollar store seasonal garland. By the time it's up in your home, it looks about 2/3 as real and costs 1/10 the price.

As I also saw mentioned, medicine. Buy it purely on price by volume and disregard the brand entirely. The only medicine I buy name-brand is Flonaise, because most generic brands of fluticasone spray have the most low-functioning applicators I've ever seen.