The source of sensor code is generally the same as the source of sensors themselves, i.e., the vendors. Not all of them supply such, and not all of them supply it in a 100 % compatible form. Note that this will be OS specific, but I doubt you will find much around for Windows IoT at this point (since it is quite a new OS) so we are talking about GNU/Linux (all flavours are the same in this sense). Presuming the sensor uses a standard interface (I2C, SPI, serial) it does not have to be hardware specific (i.e. "for the raspberry pi") since the linux API for these things is universal.
However I still think it is unusual, so yes people are commonly rolling their own code. Here's an introduction to the I2C interface; there is something similar if you look around for serial and SPI although I have not used them, and there should be versions of the APIs (I believe the serial interface is largely language agnostice) for most popular languages. Remember, you are not looking for "rpi spi", you are looking for "linux spi".
There must be some standard that people must be using...
Beyond the bus style (I2C, SPI, USB, etc) you really are into device specific stuff, so it is hard to see what special "standard" beyond this could apply. Note that some popular sensors do have drivers already available in the stock kernel, something you may want to check on.