Jasmine or juniper, juniper or jasmine? I've been going back and forth on the issue, and while juniper berry is an awesome oil, and I recommend that you buy some, I will write about jasmine, since that's what "j" is really all about. As Arctander writes, jasmine is "one of the most important, if not the most important natural perfume raw material."
One of the main reasons for writing about jasmine is that I get to write about indole, a molecule present in jasmine absolute that is also found in your poop. Indole adds a dirtiness to jasmine, along with other white flowers. It might sound gross, but don't let it scare you off. It's part of what makes jasmine complex and beautiful. Indole is also available as an aromachemical, and if too much is used, it could make your perfume gross. But the small amount in jasmine makes it interesting.
You will usually find jasmine in the form of an absolute, though jasmine wax and concrete are also available. The absolute is extracted from the concrete, leaving the wax behind. Carbon dioxide extractions are becoming more readily available, and are very lovely. I have not tried any, but jasmine ruhs are available, and these are hydrodistilled instead of solvent extracted. While rare, jasmine enfleurage oil is still produced, but it is very pricy. Each extraction method results in a different odor profile, and you may wish to try a different form if the one you're using isn't quite right for your blend.
Oh, and if you want jasmine and sandalwood in one bottle, look into attars.
If you're already overwhelmed enough by these options, you might want to stop reading. There are different varieties of jasmine available, though grandiflorum and sambac are the two most common. You can also find jasmine auriculatum in some of the above extractions, and I have also see flexile as an absolute and polyanthum enfleurage.
Country of origin can also impact an oil's odor, along with factors like the weather that year. Some oils are better than others, and you also need to be on the lookout for places that might sell adulterated oils (the adulteration can happen before they receive it).
So where to start? Buy samples of grandiflorum and sambac absolutes from a few places, pacing yourself if need be due to the expense. The samples might even be large enough for you to try them out in your blends, since you don't necessarily need much. Some places offer dilutions in jojoba or fractionated coconut oil, and these can be a great way to try jasmine for less money. If you make your perfumes in alcohol though, buy the pure stuff. Jasmine blends well with many essential oils, so don't be afraid to add a little to a fragrance you're working on. Jasmine with rose is a classic, and will add a beautiful floral note to your middle.
While nothing can replace jasmine, if you're looking for other white florals, consider ylang ylang, champaca, tuberose, and orange flower absolute.