Intermediate Must-Have Supplies
Most artists tend to acquire supplies as they go, starting with a few brushes and tubes of paint and working their way up as their time and budget permit. This is how I started, and I wrote a section for total beginners on what to get first when you can't get everything at once.
This is a list of extras which are not necessary for beginners, but are things I use every day as a professional artist and couldn't live without. I would recommend getting these things once you start buying professional quality paint.
The Sta-Wet Palette ($15-20)
This palette is magic for acrylics. I used to use disposable palette paper, and while I still swear by it for beginners, travelers and occasional painters, it results in a lot of wasted paint if you paint every day, since the paint often dries up before you can use it all. Every day, you have to squeeze out new paint, which takes a while if you have a lot of colors.
The Sta-Wet palette is a resealable plastic tray that has a sponge inside and a special magic water-permeable paper liner. When open, it will keep your paints wet all day, so you can paint for several hours without your paints drying up. But the best part is, you can put the lid on, and it will keep your paints wet for weeks. I find this cuts down enormously on my paint consumption, and allows me to use my color mixes for the duration of a painting. Depending on the humidity of the climate you live in, I recommend spraying the palette once in a while with a spray bottle just to keep the sponge wet.
A Tube Wringer ($10-20)
I use expensive paints and tend to go through them quickly, so I'm always looking for ways to avoid wasting paint. I bought the heavy-duty tube wringer a few months ago and I'm sure it's paid for itself already. It really gets all the paint out of the bottom and helps the paint tubes fold up smaller as they get used up. And, it's also great around the house for toothpaste, etc. There are small painters' keys that do the same job, but they don't work nearly as well.
The Q-Grip Picture Clip ($5-7)
Having your reference photo at eye level, close to your painting will result in better accuracy and less neck strain. It only works for stretched canvas, because it fits onto the stretcher bars.
A Brush Washer Tub ($6-12)
I held off on getting a brush washer tub for a long time, because I wasn't convinced that it would be any better than using an old jar. It's really worth the money though once you get serious. The bottom has a ribbed area where you can scrub your brush bristles, and it gets paint out of the center of the brush much better than anything else. The holes on the sides are handy for sticking your rinsed brushes in, so you avoid the temptation to leave your brushes in the water. I'm generally pretty rough on my brushes, but the brush tub has helped extend their life expectancy a lot.
Save your money...
I like gadgets and tend to buy things if they're "designed for artists" and/or shiny and cool-looking. However, not all of them are useful:
Brush holders: I have bought several brush holders since they looked so neat in the store, but in reality they're bulky, don't stand up properly and are generally unnecessary. Just roll up your brushes in a piece of cloth.
Art Caddies: Same goes for the special artist's caddy/toolbin things. They look cool, but in my experience aren't really any better than a normal toolbox/tackle box from a hardware store, they have cheaper hinges, and they cost more.
Easel Lights: Although I've never bought one (and I would welcome comments from artists who have) I'm not convinced that genuine easel lights are really necessary, since they don't seem any different from hardware-store or Ikea-type task lights with a daylight bulb. Your painting is going to be displayed indoors, not outdoors, so indoor light should be fine, right? Am I missing something?