A strategic investor is someone who has a strategic reason for investing in your enterprise; that is, they have an over-arching interest in your success. You can find strategic investors by looking through your supply chain and your value chain. Even your competitors can be a source of strategic start-up capital if they are looking to you as a new co-opetitor.
Say you are bootstrapping a new home builder. A trade creditor (supplier) might extend credit to you for building materials and supplies or a client might give you a sizable down payment on a home purchase; in essence, each of them become a strategic investor in your business. Or say you are starting an athletic wear clothing business, department stores might give you a cash advance in return for exclusivity or a sports drink company might sponsor your line of clothing in return for co-branding opportunities.