Disney tickets are rather tightly distributed by authorized ticket resellers; as such, their prices are also rather tightly controlled. The only discounts you're going to find are from those authorized resellers, or from travel agencies that specialize in Disney; those discounts are generally going to be no more than 5%-10% below the gate price.
Universal tickets are a slightly different story. Universal often has good deals going on their own web site, and the authorized resellers typically offer specialized ticket deals that Universal doesn't offer on their web site. It's not uncommon to find a week-long unlimited Universal ticket for the price of a two-day Disney pass.
My preferred ticket broker is UnderCoverTourist.com. They are local in Orlando and have the best discounts of the several resellers I've dealt with. They also handle Universal and Seaworld Orlando tickets, and tickets to many of the other attractions and dinner shows in the Orlando area. You order online or by phone. They will mail your tickets to you, or send them to your hotel the day before you check in, or you can pick them up at a UPS store in Orlando when you arrive (with advance notice).
Most hotels in Orlando have a ticket counter in the lobby, or sell tickets at their concierge desks. These are usually legitimate resellers, but their prices are no better than the gate price. Get your tickets in advance, from UCT.
A word of warning: Both Universal and Disney use a biometric ticket verification system which uses fingerprints to match a ticket to a particular person. You can NOT use a ticket that someone else has used - the turnstiles will reject it - so if someone you know has a 10-day ticket that has unused days left on it, and tries to sell it to you, don't bother. It's a waste of money. Those leftover days won't work for you.
Another word of warning: Disney tickets expire 14 days after the first use, so if someone you know has a 10-day ticket from a trip last month, but didn't use all of the days, the unused days all expired 14 days after he first used the ticket, so it's worthless anyway, even if you can figure out a way to get past the biometric system.
Both of these warnings add up to one big warning: NEVER EVER NEVER EVER NO MATTER WHAT should you buy tickets on eBay or Craig's List or from anyone other than an authorized reseller of Disney or Universal tickets, because 99% of them are intentional scams, and the other 1% are people trying to resell their leftover days without realizing that the leftover days are actually worthless to anyone who buys them.