Shows On Sale or "SoS" has been providing on-sale information and listings for a long time.
This pillar of the ticket reselling community continues to evolve its tools and has a large membership and loyal following.
The biggest downside is the cost, membership is over $150 per month (some have reported $200/mo) and you won't even find the website easily on Google unless you're looking for it.
Don't start here but if you do manage to get your business off the ground and start to really do well this is a highly-recommended place to find presales, onsales, tips, tools and a community of successful ticket brokers.
"Learn the fundamentals of reselling tickets for profits. Find out how to find the best events to buy and flip every single time using our 3 secrets to pick the hottest events. This comprehensive video training will help you get started completely for free and give you a huge head start in the ticket reselling business."
That's the promise of TicketFlipping and at first glance it looks ok, they do offer some more advanced tools, all of which come with a premium fee but seem worthwhile.
Customer service is quick to reply with help so that's a big plus.