Now, you might not be able to pick up a pre-fab St. Philomena costume in Target, and many families create their own. That's why I'm first going to direct you to Catholic Icing's extensive list of saint costume ideas for girls and boys. Lacy has a great ideas for specific costumes for different saints and ideas for making them.
Now here's where the Catholic Deals part comes in. If you don't have the time to create your own costume, or like me you're no expert with a needle and thread, you can actually buy at least a base costume for many saints. Surprisingly enough it's pretty easy to buy a saint costume if you just look for certain categories of generic costumes and then embellish them with some accessories.
So here is a current list of what I've found available in the basic categories of saint costumes. Almost all of these costumes currently cost under $25 (some quite a bit under), although prices and availability may change as the month goes on.
Knight/Soldier saints (St. George, St. Michael, St. Martin of Tours, St. Joan of Arc, etc.)
This Child Roman Soldier Costume from Autom comes in a set of four, but the costumes are just $2.19 each! At that price it's not so bad that you have to buy four, and you could always swap or sell the extras.
This Rubies Medieval Lord Child Costume ranges in price from $9.85-$16.09, depending on size.
Unfortunately, plastic armor with a cross emblem does not seem to be widely available. I did find this Armor of God Playset on Amazon for $24.99, plus $5.49 shipping. It's a little pricier than some other costumes, but it no doubt would get a lot of use as a playset after All Saints Day.
Biblical/Early Christian Saints (any of the apostles, early martyrs, etc.)
Oriental Trading has basic robes and capes in all colors and both kid and adult sizes. Prices start at just $6.25 for a basic kids' robe.
I ordered this Child Julius Caesar costume (set of 4) from Autom because both my daughters want to be St. Helena this year! I think it could work for a girl or boy.
These two costumes from Autom are available in a set of just one, and each could be used for any number of apostles or Biblical characters. The Child Jesus Costume is $4.79 for one, and the Child Nativity Costume is $4.29 for one.
Priest/Nun/Pope/Bishop saints (so many - anyone from St. John Vianney to St. Therese the Little Flower and countless more)
It's so sad that basically all the priest and nun costumes out there are designed to make fun, and are often for adults. For a basic priest or nun, I would suggest getting a basic tunic and/or veil from Oriental Trading or Autom (see links above) and embellishing it according to some of Lacy's ideas.
If you'd really like to purchase a religious costume, check our Our Coats of Many Colors for beautiful, handmade saint costumes. The prices are a little higher, but if you're looking to invest in a high-quality saint costume, this is the place to go.
Royal saints (St. Elizabeth of Hungary, St. Isabelle of Portugal, St. Margaret of Scotland, St. Hedwig, St. Henry, St. Louis, etc.)
I love this tunic from Etsy because it's custom made yet is priced at a very reasonable $15. You can also add a custom applique. This could work for a king, prince, or knight costume.
You could also add this reversible play cape to a tunic or suit of armor. It is also handmade in whatever colors you choose, and it's just $12.
Princess costumes are of course widely available for little girls, but for a saint you are probably looking for something with less tulle and more along the lines of Medieval princess.
Amazon has this pretty Medieval Princess - Purple - Small Costume for just $13.50, but it's only that price in size small (4-6).
And this Lady of the Palace Kids Costume is available in S, M, and L, and currently ranges in price from $17.92-$19.39.
There are even more ways that you can use regular Halloween costumes for saints! For example, a Native American costume for Blessed Kateri Tekakwitha, a doctor costume for St. Gianna, a shepherdess for one of the Fatima children, and the list goes on! Use your imagination, and find a saint that will spark your child's!