MechE here as well. An alternative to McMaster is Misumi. Misumi is nice because you can order parts that are made to order (imagine a rod with the specific thread and length). No next day shipping like McMaster but being able to buy a custom “off the shelf” part without making a special internal part number is huge (so you don’t need drawings or anything). They’ll ship you a couple books like the signature yellow book from McMaster. They’ll even host a seminar explaining why “good engineers design, great engineers integrate” (really only true for the industry and not consumer tech).