Reddit mentions of uxcell 5mm to 8mm Shaft Coupling 30mm Length 25mm Diameter Motor Coupler Aluminum Alloy Joint Connector for DIY Encoder

The two part shaft coupling/prune type coupler/whatever is the best! Absolutely worth its extra cost, however you have to make sure you get a good quality one. I bought one off of eBay before for cheap, and it's really not worth the extra cost.

The one in the picture doesn't look as good (due to the red pieces being much smaller than they should), but this type is so much better for one simple reason: if your print head ever slams into the bed on accident or goes too far up (because you leveled incorrectly or pushed Z too far up or down) the piece will act as a safety mechanism and actually dislodge. The two pieces will separate and will save the motor, coupler, and anything else from getting destroyed.

The stock ender 3 coupler that I had actually was destroyed because I kept on using my hand to push the X axis up, and after using too much force, the piece bent upward. It was stuck permanently until I took the dremel out to cut the screw out.

If anyone looks for a coupler, GET one of these. Most people don't think this is an upgrade, but it has saved my life multiple times.

UXCell's model is by far the best, but quite expensive. Here is the link. If you do choose to buy it, you'll actually need a button head screw for the bottom coupler piece as it will hit against the motor mount. Unlike the stock one, it is super heavy duty thick (which is why the screw hits the mount) and is quite well cut.

Haven't tried these, but I have seen them recommended a few times. https://www.amazon.com/gp/aw/d/B06X9TVW64/ref=mp_s_a_1_1?ie=UTF8&qid=1527124792&sr=8-1-spons&pi=AC_SX236_SY340_FMwebp_QL65&keywords=uxcell+5mm+8mm&psc=1

There are cheaper spiral cut flex couplers out there too. 5mm to 8mm is what you need. Can also try swapping the couplers on either side and see if the issue moves to the other side. Do some research on properly mounting and adjusting them before you attempt though.

Here's what you'd need: two 8mm x 295mm leadscrews w/ the brass nut things, clamp-style couplers, and then print any parts you may need for converting from threaded rods to leadscrews. The couplers I linked are expensive, but they work really well.

Be warned though, these parts will not help you fix any misalignment in your z-axis, they'll only help with bent rods. You need to ensure that everything is aligned yourself.

You can do this by installing the parts that hold the brass nuts and looking through the hole to align it with the motor shaft, or screwing the leadscrews in and using those to help align everything.