What are you going to power with this? Is it exactly 18V, or can it be higher or lower? (batteries are usually not at the voltage on the label)
For DIY, are you looking for a kit, or can you build it yourself?
I'm in the middle of building a power supply for a headphone amp, going to try using lifepo4 batteries. I need + and - 18vdc, plan to wire 5 batteries in series for each voltage. With batteries, you also need a charger.
How long do you want to run between charges?
For a linear PS, there are lots of chinese voltage regs on ebay you could use. I think some of them just need a transformer, and they do the rest.
Finally, there are liability concerns for DIY Power Supplies, I wouldn't do it if I didn't know what I was doing.
Seems like this might be better in the Lab forum too, at least for DIY.
I've also seen other options for linear PS's you can buy, but I dont remember the names of the places.
Randy