When prototyping metal parts, my knee jerk reaction is to get the parts CNC machined.
But there are several other alternatives:
For sheet metal goods, there are a number of quick-turn sheet metal vendors. My favorite was Rapid which was recently acquired by Protolabs. It's a great resource. Another really good sheet metal vendor is Protocase. They have template enclosure files that will meet generic needs, and can also turn around custom parts very quickly.
For custom, formed sheet metal quickly, I've successfully 3D printed forming fixtures, and then used it with an arbor press to form quick turn lasercut parts by Prototype Solutions Group, and I had custom parts in 3 days.
Also, I would totally check out Thin Metal Parts (for thin metal parts).
I use a variety of vendors for CNC machining. For quick and dirty machining, I turn to Protolabs or Xometry. They are both hard to beat from a cost standpoint. And they deliver in 1-5 days. For high-tolerance critical feature machining (or ultra fast same day service) I use Silkworm Industrial in Maple Plain, MN. Joe does great work. But he wears a beard. Don't trust anyone that doesn't show their whole face. :) I've also machined 90% net parts from Protolabs and had Joe finish them for me. Just another way to expedite the process.
Xometry offers DMLS printed metal parts for reasonable cost. Sometimes I'll order a DMLS part that's 90% net geometry and bring it to a local machinest for final precision machining.
Desktop Metal is building extrusion-style metal parts, that are incredibly detailed. The parts come out slightly oversized, and they're sintered in a special oven to burn out the binder agent and bring them down to size. Similar technology is also being introduced by the MarkForged Metal X.
Metal Plated SLA Another creative option for metal parts is to use a metal-plated SLA part. This is a very clever workaround. SLA is typically very fast, and complexity is basically free. Nickel plating an SLA part (even only a 0.004" thick coating) can make a SLA part as stiff as metal. I printed a nylon part and coated it with Nickel. It added a week to the lead time, but it came back and I couldn't tell it wasn't machined out of metal. (...except for the fact that it warmed up quickly in my hand, which was a dead giveaway). Check out Repliform for more information on metal-plated SLA. Xometry also offers this option.
You might think that die casting requires several weeks of lead time. I have a Chinese vendor that can cut and die cast custom heat sinks in 2-weeks. It's incredible. Sometimes the communication is a hurdle, so we end up doing it twice. But the cost is very reasonable. You'll definitely want to sign up for MFG.com if you want to find good qualified foreign suppliers.
Investment/Lost Wax Casting
Cheap FDM machines can print PLA which burns out cleanly in lost-wax/plastic method. Also, SLA machines like the Formlabs can print casting wax or casting resin that can be burned out of a mold and leave intricate metal forms with relatively low cost. Check out Formlabs metal casting resins.
These are just a few of a number of ways that you can accelerate development programs by thinking creatively about metal parts. Sometimes a surrogate stand-in printed part can take the place of a machined part during the chip-creation period.
Hope these ideas have got you thinking about different ways to make parts!