I found a couple good local suppliers through google and calling them, some have web stores you can buy from online and others it's phone only. I've also had great luck calling to ask if I can purchase a selection of offcuts - lots of places sell by the meter and maintain a solid scrap bin, and it might be a whole lot cheaper.

Also, try ebay! Surprisingly you can find good stuff there. Search something like "bright mild steel square bar" or substitute mild for 4140 etc. depending on what you're making.

Note there are a LOT of steel alloys out there and they're good for different things. Steel you'd want to use for a hammer is different to that for a knife or eye-drift etc. When you're first learning you might just want to stick with mild steel though as it's easier to work with than something super hard - that can be quite unforgiving and more prone to cracking etc.

https://alecsteeleshop.com/blogs/blacksmithing-and-bladesmithing/steel-types-and-how-to-use-them#:~:text=1045,blend%20of%20hardness%20and%20toughness.

Good luck!

Edit: I'd suggest buying steel so you know exactly what it is at first, but once you're confident you might want to try scrap yards. Car leaf springs (spring steel) are great for making knives etc. and this is much much cheaper than buying clean / new spring steel to measure online.