What is rustic maple?

it is also know as knotty maple

Then it's also known as cheap, low-grade maple (no offense to your tastes intended). For most purposes, the knots are considered defects. You might have better luck if you learn the lingo spoken by lumberyards. Read up on hardwood lumber grading, then start calling larger lumber dealers in your area.

Fair warning: knotty maple is going to be substantially harder to work with than clear, straight-grained material.

agree with jon270 here. Maple is known for chipping out where grain reverses and knots have this in spades (not to mix metaphors). In addition, knots will often produce severe warping and cracking in the areas around them.

There are many specialties for maple: curly (AKA tiger) maple, birdseye maple, spalted, wormy soft (WHND - worm holes not defects) (mostly considered premium grades) Also generally considered lower grades -- "sap hole", wormy (WHAD - worm holes are defects), mineral streaked, and "not sorted for color"(contains darker heartwood) .

Here is a link that might be useful: sample grade sheet

"Rustic" seems like a convenient catchall category wherein the mills can relegate all of the culled stuff, probably of diverse species of maple, and distribute it to manufacturers who cull it further into two piles, one to burn, one to make your cabinet doors. I would hope that the faceframes are a better grade. It's going to contain a lot of the character marks/defects enumerated above.
Casey

My local dealer has a cart of "shorts," usually 3' to 4' long pieces of cherry, walnut, and maple. The sawmill cuts these off to increase the grade of the rest of the piece, and until my dealer decided to buy them, the mill burned them. They run about 20-30% of the price of Sel&Btr stock. At that price, I buy them, trim off the bad parts and have lots of stock for repairs. I hate putting cherry inside a sofa, but at $1.50 b/f, it's the cheapest stuff I can find.