@Tespo a good info resource is https://www.mcmaster.com/screws/ you can browse around to see what you want and what exists bolts and screws are given like M5x20 (metric, 5mm diameter, 20mm long). socket head screws are used a lot on printers. or maybe rounded head screws. common t-nuts https://www.mcmaster.com/screws/structural-framing/nut-style~hammer/ those assorted packs you mentioned are actually more handy than specific lengths, because you'll have a lot of sizes to choose from and see what fits best, especially if you will be making more things. i've had good luck with this store https://nindejin.aliexpress.com/store/5053209