you can get waterproof wire nuts at home depot or lowes. they are wire nuts with grease in them and found on the irrigation aisle. or, what I do is get a tube of dielectric grease at same stores and squeeze some on twisted wires and in wire nuts- works great.
You can get grease nuts from home depot or lowes in the irrigation aisle. these are for direct burial wire connections
I'm not sure what are waterproof wire nuts. I used the following method with success. The insulation is stripped off the ends of the wires. The wires are twisted together and the wire nuts are screwed on. We then used black electrical tape to wrap around the wire nut and wire. Lastly, we painted liquid electrical tape on the assembly to waterproof.
If your still looking on an answer, Google 'low voltage waterproof wire nuts' to find sources. A good hardware store should carry them also. They work great. I use them for all my landscape lights including ones in our water features. Hold on to your hat though, they will cost anywhere from 75 cents to 1.25+ EACH.