If you have an ethnic grocery store nearby they usually carry it fresh rambutans, in season, and at a reasonable price. I usually find it for $3-$5 per lb. I'm not sure if the trees are dioecious or not.

There is an ethnic grocery store that carries Rambutan, but it's canned instead of fresh. I did buy some Rambutan when it was in season, but I cut into the seeds when I was cutting the fruit, so I'm not sure if these would have grown.

Why did you use a knife on them to begin with? You can split the skin with your fingers. The spiky things are soft and I just use my teeth to remove the fruit from the seed. I doubt that the seeds store well as they don't have a hard seed coat. I've planted them for fun after eating the fruit and they germinate easily as long as they are kept warm.

I also believe that the easiest way is to just plant seeds from fresh fruit you eat. but if you have no access to them.

here are some sources:
http://www.simply-thai.com/Thai-Market_Fruit_Rambutan.htm
http://borneocollection.netfirms.com/BorneoSEA.htm

note always a risk ordering seeds from outside the US ( customs ) but i have ordered from simply-thai and did get my seeds. ( papaya variety from asia )

You could try here. I just ordered some yesterday.

http://www.fruitlovers.com/seedlistUSA.html

99 Ranch market had them on sale for 3.00 in Rancho Cucamonga in Calif. Check to see if you have that Asian Market in your area.