Venture capitalists (VCs) will invest in businesses both at start-up and at times when the company needs a cash injection to grow. If you're looking for VC funding, make sure you look for a group that has history of investing in music related businesses. Although VCs look for high risk investments, they're not always good matches with creative industries unless they're used to that realm. In other words, if you can even get them to take you seriously to begin with, they don't really care about your "artistic integrity" - they want the loot, and they want it fast.
Americans can all but forget about this one, but outside of the States, most countries have funding bodies that provide money for the arts, including the music industry. These arts funding groups can be great places to get the money you need because they are willing to work with music businesses of all sizes and have the ability to take chances on projects profit seekers like angels and VCs wouldn't touch. Even better, most of the time they give grants rather than loans, so you don't have to pay it back. You'll still need a good business plan to work with them, however - though in most cases, they can help you write it.
For indie labels, investment by a major label is an option. This kind of investment will typically only after you've built a proven track record of success as a label and need money to expand - start-up cash from a major is usually only given to someone who has either run a successful indie label in the past or has a good sales record as an artist.
Of course, funding from a major will require cedeing some control of your label, which has not always ended well for indies. Learn more: