Renoise has no native synths. it is really sample-oriented. as satobox pointed out, as of 2.6 you can use scripted tools, and one of the best is the ‘Generate Custom Wave’-tool, which lets you do just as the name implies. of course, this is then a sample of a waveform, instead of an actual synthesizer. however, you will find that with the tools at your disposal, you will be able to do a lot of stuff to just a single-cycle waveform to make it sound like something fantastic. the same goes for samples: truthfully, there are so many out there that your fear of using the same stuff as everybody else is not very realistic. that said, instead of downloading, you can always record your own for example.
also, you might want to check out dblue’s ‘native monophonic synth’. it’s a neat trick, and it works pretty well.
i hope you will give Renoise the chance it deserves, despite the lack of native synths. if you are really in need, indeed use VST.