Indio Mandarinquat are often found at Home Depot and Lowe's in 5 gallon pots, sold at about $21 - $23. Some times WalMart carry these trees also. It is a bit harder to find than Lemons, Valencias and Washington Navels.
If you want to plant the tree inground, buy the smaller sized trees, they will grow better, and often outgrow those that you planted from bigger pots. My preference has always been to buy citri-pots, the ones that are sold for about $7 to $11 in square tall pots ( 4"x4"x18" ). The next best thing would be the 5 gal pots. I would only buy the 15 gal pots if that is the last size that have the cultivar that I want.
But for citrus trees maintained in containers, I go the opposite way, the bigger the container trees, the better.