While the best way to discover startup ideas is to find a solution to a problem that you personally face or with a subject that interests you, sometimes you just don't have the luxury of waiting and need a startup idea now. Like when your existing startup fails to generate traction beyond a pivot.

Inspiration doesn't always then strike when you want it to. You can't necessarily force startup ideas and sometimes, they strike you in the unlikeliest time or place. But if you've got your heart and soul pining to start the next or your first startup, and you're starved for ideas, look no further.

These four resources don't just provide inspiration or food for thought, but some come with an execution plan and some with an investor backing it up.

1. Paul Graham's Startup Ideas. Paul Graham has been at the helm of the industry's most prominent accelerator. Inc, in an interview, describes him as, "As co-founder of Y Combinator, the Silicon Valley tech accelerator, Graham has made a career of turning half-cooked business ideas into fast-growing companies in a matter of months."

In one of his essays, he talks about some frighteningly ambitious startup ideas, and goes on to further state and anyone of these could make the entrepreneur a billionaire.

Now, you may not want to take a head-on battle with Google for your next venture, but the fact is that all startup ideas are frightening till they achieve product/market fit and grow exponentially. You don't necessarily need to make those tall claims on day one of launching your startup, but you can certainly work your way up to the point that it can challenge the norms.

From a new search engine to replacing email and universities, here are 7 of Paul Graham's most ambitious startup ideas that can make you a billionaire.

2. IdeaMarket. Bill Gross, the founder of the technology incubator Idealab and 100 companies in the last 30 years, also co-founded IdeaMarket. IdeaMarket is a curated crowd-sourced marketplace for ideas that solve everyday problems and uncover interesting new opportunities.

Ideas are matched with talent and money to launch companies. In simple terms, this is a place to find ideas that are already backed by investors, with over $4.2 million in funding available across 35 ideas, as of today.

All you need is a solid team and an execution plan to launch any of the ideas currently listed on their website. Choose the one you're most passionate about solving, present your team along with the business/execution plan and the winner takes home the funding mentioned alongside the idea. To get started, take a look at the current startup ideas.

3. Buy a website. Did you know that there exist something called website brokers; people or companies that buy and sell websites? This is a great place to look for not just inspiration, but buy into an existing product that is consistently generating revenues.

Neil Patel of KISSmetrics recommends buying only the assets and not the entire company (to stay away from any existing liabilities). He also mentions that people who want to sell their existing products usually have three common qualities: a) they don't update their site too often, b) they don't make too much money from it, and c) they don't care about the business anymore.

If you can spot these three qualities in a product listed, you can get a good bargain. But, the foremost reason for purchase should be an idea that you can grow into a much larger business, aka, something you're passionate about.

There are many brokers on the market, but here's one to get you started (also recommended by Neil Patel).

4. Leverage your skills. Many ways to do so, but here's the best way to forge ahead into building your startup even when you don't have an idea or a team. Many startup launchpads have come into existence that invite people from all walks of life, which have an idea or not but want to be part of a team building a product.

Typically, these events run into 2-3 days and over this course of time, you meet with several interesting people with different backgrounds, some of which have a startup idea. These events encourage people to team up with complimentary skills to build a prototype, a business model and eventually to launch the startup.

So if you're a marketer looking for a tech co-founder along with a startup idea or vice versa, register and head to these events to get started. Not only do you get a chance to discover a great idea to work on, you get a team or a co-founder and mentoring to take your idea forward. Here's one to get you started: Startup Weekend, powered by Google for Entrepreneurs (I mentor startups at Startup Weekend).