Are there any types of DC low voltage power supplies on the retail market that can accommodate for long wire runs (18 gauge wire). I am worried about voltage drop and power failure; I'm trying to power window shade motors with 18 gauge wire over long distance wire run and I need 18 volt output.
Generally, when you need to tightly regulate the voltage for a variable load at some distance from the power supply, you look for a power supply that has "remote sense" terminals. These allow you to create a 4-wire connection to the load, and this allows the internal regulator to compensate for the voltage drop in the main current-carrying wires.
However, you may be over-thinking the problem. AWG18 wire has a resistance of 20 mΩ/m, so a 25-meter run of 2-conductor cable has a total resistance of about 1 Ω. Even with a motor drawing 1 A, this would only create a voltage drop of 1 V.
If all of your motors have about the same length of cable running to them, it may be sufficient to simply turn the voltage of the power supply up by 1 V to compensate for the cable drop. Most pre-built power supply modules have an adjustment control for just that purpose.
That's a worthy concern. You need to google the resistance of 18ga wire and do the Ohm's Law calculation. It's quite straightforward.
Keep in mind that 14 and 12 gauge wire are unnaturally cheap because so much of it is used in architectural wiring. Compare the prices of 12 and 14 gauge NM (Romex) multi-wire cable, and 14 and 12 THHN/THWN single-wire. So in your particular situation, "throw copper at the problem" may be the way to go.
The best answer is do the 120V to 18V conversion as close to the point-of-use as possible. One option is relays. Energy loss is the square of current, so wires that have too much voltage drop for full current can easily handle a small relay-coil current.