All you need is one of each. For the most part citrus are self pollinating. For example, I wanted to buy a honey bell tangelo but the description said that it did need a pollinator.

You might check at your local Home Depot or Lowes. They sometimes have them. I keep mine in containers and they produce very well.
If I might suggest an orange it would be of the type Satsuma. I have 3 different varietise and really enjoy them. They are a lot like the Cuties you buy by the bag in the grocery store. They are very sweet and the peel falls right off of them. I have a MIHO, a Brown Select, and a Owari.
For a lemon if you want one like at the grocery store it would be a Eureka, or a Lisbon. There is also an Improved Meyers that is a cross between a lemon and an orange. f you let them ripen fully on the tree they will be a lot sweeter than a regular lemon.
As far as ordering on line several places are very good. Four Winds is good as are several others. google for citrus trees and see what you come up with.
When you repot them make sure that the potting mix will drains very well.

Hope he enjoys them,
Charles

Another consideration is the blooms. Meyers seem to bloom prolifically and the smell is wonderful. The tree can be a bit fussy--search winter leaf drop and see--but the fruit is good and mine blossoms around Christmas and then periodically throughout the summer. Fruit takes forever to ripen indoors for me, even with supplemental light in the winter.

I do not know about mail order, but a lot of the higher end garden centers have citrus, even here in MN. I stop by that section just to smell the blossoms. Some are better than others. If you have a fancy garden center nearby, call to see if they have any. Places around here just started to get theirs in for the season. The small (1 gal.) lemons and limes are reasonable, but everything else seems to be a little expensive.

Another way to find them is through the Monrovia website. Pick a plant, enter your ZIP code, and they can tell you which places nearby Monrovia has shipped to. They cannot tell you if it is still in stock, though. Of course it is limited to Monrovia stock, but it will at least tell you which places might have some other citrus as well.

Eric