You probably didn't get many responses as this is something even the pro's deal with frustratingly. Where to get designs. It is a motivation to gain the graphic skills yourself when it is as difficult as it is to find artists that "fit."
There are canned packages of clip art many pull from. Groups like Advanced Artists and others provide packages that jump start some designs. There are many freelance sites out there. A web search on freelance graphic designer will get you a flood of hits, with Fivver being loved and hated alternatively. The best designers plainly cost more money. They can be hard to justify the costs unless your making hundreds of shirts off a design.
Basically you beat the weeds and bushes looking for some one that matches your tastes, workflow and budget, or learn the skills yourself.
You should definitely look into finding a good graphic designer. One who understands layout, color theory, has historic motivation, and can illustrate designs from hand before going to the computer. That person must be able to create a line of t shirts instead of a one hit wonder.
A small part of my previous career involved doing some work with CorelDraw and PhotoShop, and I also messed about a bit on my own--so count that as several decades of light messing about with graphics software. I've since upped my skills quite a bit, but only because I pushed and tried to do things I didn't yet know how to do, but learned in the process of doing.
The internet is a great tool. When I'm playing around with a design idea, I Google and look for inspiration in terms of composition and drawing style etc. However, too often I find something and think: "This is freaking PERFECT for my design!" ... or near enough anyway, that I am tempted to directly incorporate it into my design rather than merely find inspiration in it. My point being, the WWW leads to laziness.
Yes, sure, I have paid for rights to some photos that I have used in my designs, but I try to avoid that with clip art type stuff, and create that myself.
One way to get better with a design tool is to down load a graphic that you like and put it on one layer, then recreate the graphic on the layer above from scratch. An exercise like this is a great way to push past your current skills and learn new techniques. In any case, realistically you'll need some graphics skills to make use of any graphics you did buy, so best to start working on it.
Beyond the art, there is the subject matter. What niche will you be targeting with your designs? You can't be everything to everybody. Pick niches that you know and understand so you can create designs that connect with your target, all the better if it is an underserved niche beneath the notice of the big players.
NoXid is right. The the internet is one that encourages copying. However now a days I can come up with a design based off the subject. My early days was me completely ripping somebody off. @NoXid if you are still struggling for style, which I think you're beyond that, you might want to spend some time in the malls. Or at least let me say, you should go where your audience hangs out and look at their art. So for me, my niche watches anime, hangs out at skate parks, might buy from urban outfitters, zumiez, etc. They'll look for music videos with a wide range of colors. This is where I live in.
I'm thinking designers are now leaving the wide range of art styles and are buckling down to 1, unless it's about brand messaging. But if you are going to work with a designer here is my advice...
1. Don't send him or her pictures of other people's work. Send them pictures of the environment in which your audience lives in.
2. Ask them about the process. How do they work from start to finish..
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