You could take some ideas from methods used by creatures to swim without fins.
Sea slugs undulate their flattened bodies to move along thru the water. It does not look very efficient but these things have been around a very long time so they must be doing something right. Your dragon's body could be very flat. A flat thing will fall slowly if flat side down (e.g. a piece of paper) because of air resistance and the dragon capitalizes on this. It undulates its body, pushing against the air to propel itself. Such a creature would orient itself flat-side down most of the time but like a piece of paper would be capable of very rapid edge-on swooping dives. This would be more of a Chinese dragon than those Game of Thrones types, I think.
Squids swim this way.
Perhaps the most common type of locomotion used by cephalopods is jet
propulsion. To travel by jet propulsion, a cephalopod such as a squid
or octopus will fill its muscular mantle cavity (which is used to get
oxygenated-water to their gills) with water and then quickly expel the
water out of the siphon. The force of the water jet coming out of the
siphon is opposed in equal magnitude by the force of the cephalopod’s
body as it moves in the opposite direction (Newton’s 3rd law). These
equal, opposing forces send the cephalopod jetting away from its water
stream, much in the same way that a rocket ship is sent in the
opposite direction of the exploding rocket fuel coming out of its
Your dragon would gulp air and then expel it forcefully, propelling itself along. Something like this could be a good addition to the undulating flight for when rapid motion is needed. Dragons traditionally can make fire which is hot air - expansion is a fine way to make pressure although a jet of flame shooting from the rear might be as much funny as awesome.