where to buy DNA sample kit? why isn't it easy to find online?
Most people using these tools for research and diagnosis have long-established connections and providers of industry standard tools. Most companies that provide these tools need a steady, reliable source of income, and historically demand for those tools comes from facilities such as hospitals and labs, not DIY biotechnologists (who typically can not afford the tools), so they focus on getting their products to the population demanding them most. There is budding interest in providing research and development tools to the general public such as this startup's stated goal:
http://www.the-odin.com/about-us/

But generally you have to go to a company's website, select the product you desire, and request a quote based on how many kits you'd like to order. The process isn't as easy or accessible for DIYers simply because the demand has not been great enough until recent technological advances such as CRISPR and DNA sequencing technologies made it cheap enough to run these tests on home equipment. It's similar to how not every computer geek could afford a CPU of a certain speed 20 years ago, but now CPU's of that speed are wide spread.

Salimetrics has some swab kits available for under 100 USD here
http://store.salimetrics.com/saliva-collection-methods/

Oragene offers saliva collection kits and this link details some storage options. 
http://blog.dnagenotek.com/blogdnagenotekcom/bid/40944/Best-Practices-for-Long-term-Storage-of-Oragene-DNA-Samples

A list containing price per sample of oragene sample kits can be found here:
http://genomics.pitt.edu/pricing/price/HSCRF%20Genomics%20Research%20Core%20Price%20List.pdf

Some issues with collecting a saliva sample for DNA testing are neutralizing the enzymes and materials present that will degrade the DNA, and then isolating, storing, and testing the DNA. These techniques are usually used by trained professionals, once again highlighting the reason companies do not focus on selling to individuals, but to professional organizations.

An overview of the process of saliva collection and links to a few providers of the tools required to acquire your sample can be found here:
http://www.the-scientist.com/?articles.view/articleNo/43347/title/Tools-for-Drools/

This is a handbook detailing some tips on techniques used for collection and storage:
https://www.salimetrics.com/assets/documents/Saliva_Collection_Handbook.pdf

I am not affiliated with any of the companies mentioned above but if linking them is a no-no here on Stack Exchange the links will be removed and I will mention the companies only as examples. Hope this helps!