There are small fans that are used to move air between rooms and are hung in the corners of doorways. The fans about 6 or 7 inches square (including the housing) and generally come with a variable speed control (like a dimmer for lights) so one can turn them down to very slow. They are very quiet at the slowest speeds, a little whine-y at top speed because the diameter is so small. All though they are often mounted in doorways, they can be perched anywhere you like.

They are often used in households with wood stoves, to spread the warmth. Try a store that sells stoves. Depending on where you are, they might also be available at HD or Lowes if they have a stove department - not all stores do.

Search Home Depot for

Entreeair (door frame fan) 19.95

and

Thruwall (thruwall transfer fan. 54.95

I use AC mini box fans from northerntool.com. They are $6.99 a piece. There are two sizes; 40 CFM and 100 CFM. The 100 CFM is too much flow. They are used and just come with clipped wires so you have to make up the connections yourself. Since they are used, you may need to buy an extra one or two in case you get a noisy one. I also bought their portable thermostat; $39.99. I hung two fans with hooks made from wire on the back of each radiator. I then wired all of the fans to the thermostat. I used the holes in the floor for the radiator lines to run the wires through the basement to the thermostat. I also protected the wires from the hot piping with a bit of aluminum foil. The sensing coil from the thermostat lays on top of one of the radiator supply lines in a hidden area. When the heat comes on, a short time later the thermostat turns the fans on. After the heat shuts off, the fans turn off 30 to 45 minutes later. You have to play with the thermostat a bit to get what you want. It made a big difference in responsiveness of the radiators.

Here is a link that might be useful: Northern Tool