Mary Jane shoes with low heels/thick heels are actually fairly easy to find, as I've been able to find them at stores such as: Payless, Sears, Target, Ross etc.
Now issue is, 99.7% of what you'll find will be black, which is great since you said you're looking for black now.. but not so great for when you need other colors. Unless you can wear children's sizes, chances of finding light pink or pastel blue mary janes is next to nothing.
For that, I'd recommend making your own. You can get a pair of plain mary jane shoes, and then paint them with paints such as Angelus Leather paint (which works great for shoes and comes in a variety of colors) You can also get adornments like bows, flowers, etc at stores like Michaels to make your shoes cuter.