Coming from the buy-side, I'm sure I have a different spin than those sales & marketing gurus who I'm sure will be keen to help you & comment shortly.

You're lucky because there are SO many avenues these days. However it is hard to keep on top of them all and there's no one stop shop currently. Having worked in the public sector, I can help provide you with some of their advertised opportunity / tender sites. These tend to be very high value & very competitive but they also are well advertised in advance (unlike the private sector). There's also a few private sector customer/supplier portals around in recent years which link people together to obtain/provide this expertise. I'll be happy to point you in the right direction of some I've used & discovered on a call, if you wish.

To find individual projects I might also suggest starting with freelancing sites like Upwork (recently rebranded from oDesk), Freelancer, Elance, Toptotal (for developers), Crew, Get A Coder etc. Whilst they tend to be more bespoke, they're getting more prominently used by larger companies now.