Unfortunately it is not hamfest season (not in this part of the US anyway), but I have often seen nice linear power supplies at them. Many of the 24 volt supplies can be adjusted down via a trimmer pot, not sure if you can get to 18 volts or not, it would depend on the model. There are some well made switching supplies with relatively low noise outputs, Vicor comes to mind. Expensive new, but again pretty cheap at hamfests.
As Stretchyman said, many laptop supplies are around 19 volts, and again, they vary in quality. A genuine name brand supply (say via eBay) is most likely to be low noise, OTOH the Pride of China supplies are almost guaranteed to spew RFI. I ran into that problem when I got a replacement supply for one of the kids' laptops. S9+20 noise over all of 43 and 48 meters. Yuck.
A linear supply is going to be larger and less efficient, but guaranteed to be low noise.