When it comes to items like fixtures, door knobs, and even cabinets, you can often find less expensive options on your own than through your contractor.
Most contractors will work with you, if you want to source some of the materials yourself, just make sure you agree before the project gets started. For example, if you're building or renovating a kitchen, you might be able to find cheaper counter tops at home improvement stores than through your contractor. You can also look for reclaimed materials to use in your project. If there's a Habitat for Humanity ReStore near you, it's worth a trip to see what they have. You can often find gently used elements like shelves and fixtures there for a deep discount.
Don't wait until the project has started to decide what you will and won't be sourcing on your own. Instead, sit down with your contractor before the project starts, and go through the budget line by line. You want to make sure you know when they will need each item on site, what the measurements need to be, and what the labor costs cover, if they are putting anything together for you. In the bathroom that we added, an IKEA sink base would have cost less to buy, but the labor to put it together made it more expensive than the one our contractors sourced.