There are four Disney outlets in Orlando. I think you can find directions to all four at
www.allearsnet.com. There are two at the Belz outlet mall, one at the Premium Oulet Mall off of 535 (can see from I-4), and one at the Lake Buena Vista factory stores off of 535.
My personal favorite is the one at the Premium Outlets. The store is much nicer, and they have more current items (IMHO). The outlets are hit or miss for souvenirs. I have seen some great deals. Most of the items there are either out-dated (i.e. 2004 Memorial Day t-shirts, when the holiday has past) or old (a hat with a previous version of a resort logo).
These outlets are "official" WDW merchandise. There is also a "WDW Store" at the Crossroad shopping Center near DTD (as well as the Orlando airport), but the prices are not discounted.
WDW used to never have "sales" on their merchandise - they would send them all to the outlets or the cast store, but I am finding that they are now running sales in the parks and at DTD.
If you want less expensive general Disney merchandise (not park related), check out
Walmart (as previsouly suggested) or any number of the "tourist" stores for Mickey t-shirts. They have a ton of stuff.