If you only have time to Mardi Gras shop at ONE place, I totally recommend Party City!!
They have a whole wall of Mardi Gras decor, costume, and party items!
The prices were pretty decent too!
I bought 2 packs of 8ct Beads $1.99 each
Mardi Gras Sign $.59
Mardi Gras Top hat $1.99
Mardi Gras colored coins $5.99 (kind of pricy, but the other coins had clovers on them and I wanted the purple ones!)
They had cups, plates, tablecloths, masks, big signs, little signs, such a great variety to choose from. I got quite a bit for less than $20, and then I spent another $8 at Dollar Tree before I went to Party City.
I bought tablecloths at Dollar Tree, but I actually liked the one at Party City better, but I just stuck with what I had.