Hey Nam,
So there are a ton of different ways to finance deals, but I'll throw out a few ideas for you. For the record if you haven't read The Book on Investing in Real Estate with No and Low Money Down, I highly recommend it. It isn't go to be the end all be all of real estate financing books, but it does point out and explain some great strategies you can use, especially once you've tapped out of conventional mortgages.
1) Partner's - If you have not already formed an LLC for your rental properties I would consider asking an accountant if you should. If you have then signing on a new officer and drafting an operating agreement really isn't too hard. Partner's are great because you have trust and rapport when you bring someone on as an investor. Another possibility (Disclaimer: I'm not a lawyer) is to form an entirely new entity, separate from your current business, and find someone who wants to finance some deals. Your corporate office is probably the first place I'd suggest.
2) BRRRR- if you listen to the podcasts I'm sure you've heard this term. It stands for Buy, Rehab, Rent, Refinance, Repeat. So this would be a deal that you would flip and then rent instead of sell. You'll have to acquire these with hard money, private lending, or some other non-conventional loan as their usually in too bad condition to have a traditional mortgage. Once you've fixed it up and rented it out for 6 months - 1 year, you can find someone to refinance it under a traditional mortgage (do that ahead of time) so you can ditch the hard money or whatever loan you used to acquire it. These can help you get around the debt:income ratio because you usually have an equity bump during the Rehab phase (thus decreasing the debt on the house).
2) Private Lending- this is exactly what it sounds like. Individual with a lot of capital will lend to flippers or landlords such as yourself. Think of this as owner financing for acquiring investment deals instead of retail deals. These are more expensive but if you're out of luck with conventional mortgages its one avenue to consider.
3) Lease options - Make sure you really do your homework and understand the financing before using this one. Brandon Turner, in the book mentioned above, bought one of his first deals through a "master lease option". This means you'd lease a multi-family property (which you may potentially then sub-lease for a little cash flow) with an option to buy at a predetermined price at a predetermined time. This allows you time to see if the property cash flows correctly and if you have any problems with tenants, while also finding financing.