You might have seen it already but just in case check out this thread
https://www.reef2reef.com/threads/get-any-part-you-need-right-here.321961/
There are a couple of other 3D printer threads on Reef2Reef as well.
As for designs, maybe try designing your own? You can browse thingiverse (there is a couple of frag racks on there from memory) or other images for inspiration and then use Fusion 360 to draw it up (its free and can be really easy to learn the basics with a couple of quick youtube tutorials).
At the end of the day improving CAD skills when you own a 3D printer is really handy. The first few things you print might already have designs out there. But then one day you will think of something you want to print and you wont be able to find a design and will need to draw your own. By drawing your own simple prints (like frag racks) as practice, you will get decent at drawing basic CAD designs in no time. It can be a little time consuming at first, but it's pretty dang satisfying going from no CAD/3D experience to literally holding an object that you designed and printed out yourself.
As for the material, each manufacturer and material are all slightly different and some might be ok and others might not be. PETG and ABS are usually reef safe. To be extra safe try to stick to natural, white or black colour pigments as I have heard that some colour pigments can leach (but not certain).
PLA has been getting mixed reviews as far as reefing goes and I am having a hard time figuring out if it is fear mongering or not so I am giving it a go to see if it degrades or not.
PLA is biodegradable but as far as I am aware it needs quite specific conditions and bacteria for that to happen. There are a few people who have used it for a couple of years or more with no issues (some here on R2R). And then there is a lot of people who insist it breaks down in reef tanks, however I have not yet seen anyone say that from first hand experience. So you can use PLA, but do so with caution (don't use it for anything essential and check on it every now and then).
Personally I am trying out PLA in my tank with a feeding ring and a nitrate reactor. I'll also be printing out some random flow generators some time in the future (again in PLA).
There is HEAPS of uses for 3D printers and reefing, you just have to let your mind go wild
Projects I am currently in the middle of / on my to do list (almost all I will be trying to draw myself)
- Reef-Pi Aquarium controller housing and power boards
- Housing for a DIY skimmer neck cleaner motor
- Random flow generators (using someones design on thingiverse)
- Holders for power head controllers and power bricks
- Screen lid corner brackets
- Skimmer stand
- ATO float switch holder
- Dosing tube holder
- Probe holder
- Frag racks
And I could go on
