It sounds like you are looking for Lego Classic[0] which are pretty much just buckets of bricks. I have seen them in stores although not as prominently featured

It sounds like you are looking for Lego Classic[0] which are pretty much just buckets of bricks. I have seen them in stores although not as prominently featured as say the Star Wars sets. The Creator 3 in 1 theme[1] is also made up of generic pieces. Although they do come with instructions for three different models it’s also pretty easy to use the parts to make your own designs. Finally, there isn’t anything stopping you from buying a bunch of sets and dumping all the pieces into your own bin or bucket. That’s how most of my family’s sets ended up anyway after our sets got scrapped for parts to make new things. [0] https://www.lego.com/en-us/themes/classic https://www.lego.com/en-us/themes/classic [1] https://www.lego.com/en-us/themes/creator-3-in-1 https://www.lego.com/en-us/themes/creator-3-in-1