An avg person's saliva is usually "slightly alkaline at a healthy pH of 7.4"
A pH of 7.5 is "preferred pH of sperm (Basic) to counteract the acidity level of the vagina."
Edited to say: even though this doesn't look like a big difference, there are instances where your saliva may be more or less alkaline, as Adina stated. I have also read on several occasions that saliva is not a sperm-safe lubricant.
You can order Pre-Seed on the internet. It's not sold in stores here, either, but it seems to bring success to alot of ppl.
Here's a link if you're interested: http://www.babyhopes.com/pre-seed.html