Vaginal fluid is the best but it often isn't enough. Following that, saliva is a debatable second place but it dries fairly quickly. If you find that you need to use a commercial lubricant you want a water-based lubricant. There are hundreds of different manufacturers many with different features. I'd suggest you go to the most comfortable sex shop in your area and talk to the clerk about different lubricants, what seems to be popular and what their opinion is. When you work around lube all the time you get to know a lot. Buy a few sample sizes and try them out, go with the one that seem to work the best for you.
Sexual lubrication or arousal fluid is blood plasma that is squeezed from the cells in the vaginal walls when you get aroused and experience vasocongestion (swelling up). It will often differ in different phases of your cycle and your life and may also depend on how hydrated you are.
Sexual lubricants are often hidden as 'personal lubricants' and can normally be found with the condoms, pregnancy tests and yeast treatments in the same isle of your store. Some also have a warming effect which is said to make the clitoris and penis much more sensitive and arousable.
If you are trying to have a baby it is important to have a sperm friendly lubricant. Normal sex lubricants and saliva traps sperm (it does not kill the sperm but makes it difficult for it to escape up the cervix). Pre-seed is one such a sperm friendly lubricant.
As W0LF said, water based is by far the most preferable ones. (The opposite is glycerin/petroleum/oil based lubes that are just plain nasty to condoms and difficult to wash off) If a water based lube dries out, you can add just a few drops of water and you are in business again. Water based lubes also plays well with sex toys and condoms if you are not trying to conceive.
There is also a spot at the top end of your vagina (called the 'deep spot' or 'X spot') that researchers are thinking can control lubrication. If your partner have long enough fingers you can try it. It is between the cervix and the top front (belly side) of your vagina. Researchers have seen a massive increase in lubrication when this area is stimulated.
If he is using a condom, a squeeze of lubricant inside the condom can make it much nicer for him. It will make it feel much more natural for him and you and it will conduct heat much better.
Some lubes are also flavored but I am a bit suspicious of it. Any sugars in your vagina will cause yeast problems so I just don't trust these.
Best of luck with your quest! You will be amazed by how much fun a good lube can add to your sex life.
The best lubricants aren't always the ones from a shop - many places around the world don't have specialised sexual lubricants readily available and many budgets won't stretch to the expense of specialised lubricants...
Sometimes the best lubricant is what will do the job at the cheapest price or simply what is the most handy and/or easily available.
Sometimes a lubricant needs to appear to have another use to save embarassment.
The choice is made easier if latex condoms aren't being used because the door is then opened to grease or oil-based lubricants.
There is little wrong with oils and greases and according to doctors their use can prevent some minor infections...
The kitchen can be a source of budget-priced lubricants and butter is probably one of the most traditional things to be pressed into service in this way along with a variety of oils and fats depending on the social status of the user and what was most plentiful and handy...
Homemade oil/water emulsions of various types can be experimented with and can be produced in the kitchen blender.
Coconut oil seems particularly well suited to this application and has moisturising properties also...
Petroleum jelly also has sexual lubrication properties among its many other uses and appears quite at home in almost any room of the house and so doesn't attract attention or ribald comments - a few drops of water or some saliva over a smear of vaseline becomes very slippery!
Certainly the whole world is not in a position to run to the shops whenever something is needed and there are still many who either cannot be a slave to the retailers or wish not to be such slaves!
Most people, most of the time, only need additional lubrication occasionally so it makes little fiscal sense to buy expensive specialised products before readily available and cheaper alternatives have been tried and rejected...
If money is no object I believe the new silicone lubricants are considered superior.
I certainly cannot condone using a lube inside a condom because of the danger of the penis beginning to slide in and out of the condom rather than the penis with its sheath sliding in and out of the vagina - this is when many condoms tear and cause many unwanted pregnancies.
It is mostly for the reason of tearing in this way that condoms are only rated at just over 70% effective as a stand-alone type of birth control. The condom should be unrolled over a dry penis and should the amount of pre-cum make the inside slippery enough for the penis to move around inside the condom it should be replaced!