I posted this response to a similar question here:
An investor property line of credit is definitely harder to find than a mortgage/term loan. They generally do not come with terms as friendly as a Home Equity Line of Credit (HELOC).
When I sold and underwrote these types of facilities at a large regional bank, we generally limited the offering to strong relationship clients. Even then, we would charge unused fees to ensure that we collected enough income so the deal made sense. Lots of investors like having the capital available and never use it. Consequently, they never pay interest. This is great for the investor because they pay only if they need it. No so great for the lender who still has a commitment and isn't collecting interest. The bank still has to reserve capital for the potential usage and it costs them money.
The downside for the investor is that most lenders limit the term on their lines of credit between 12-36 months. This means that to renew the facility, the borrower must go through the underwriting process every 12-36 months. Depending on the size of the loan, this could mean paying for new third party reports like an appraisal. Some lenders streamline this process, but it can still be a hassle. You can mitigate the appraisal/third party costs by providing a negative pledge on the would-be collateral property and getting an unsecured line of credit. This may be only available for high net worth individuals with strong balance sheets.
Ultimately, I would call around to local banks and credit unions to see if they have an offering. The less standardized the loan product, the more variance you will see in offerings. As such, calling around will get you the best deal.