We know about thingyverse, as do most people with 3d printers.
There are some useful things on there and I am sure people are happy to search it. There are a few things to watch out for, as some of the designs look great as a 3d model but suck in use. Also if you use a printing service, be aware that Thingyverse models don't take shrinkage or different filament/head combinations into account, so be prepared to tinker.
To look at your examples so you know what to look for.
Sledgefire shells- Justajolt sells Sledgefire shells very cheapley if people want robust working ones. Often printed ones fit very badly at the base and dimensions are very important.
Pump grip- Printed side rails require a very clever I beam design to work properly and good use of infill. This design is large and will be expensive. Gavinfuzzy sells a better pump grip.
Fun, but won't work with UK standard light switches.
"It's only a model." No sign of a working printed one- a common problem.
3d printed "masterkey" brackets always break, that's why nobody uses them. Side walls of the clamps are particularly snap prone.
That is a stupid idea, contravenes rule 1 and forum code of conduct. Clearly designed by a twat. Dangerous mods do not belong on this forum.
Magstrike owners- good luck getting a working one of those printed "cheap" as they are very complex, requiring a lot of printer time.
In short, 3d printing isn't the best solution to the problems you will encounter in Nerf. It can be part of the solution, but always treat other people's untested, unprinted models as suspect until you can see they work, as print time can add up to a very expensive part that doesn't work otherwise.
Edited to be less hash. Tea:Patience ratio was off.