For those of you who are handy with a soldering iron and are on a shoe string budget like myself, eBay is a great source for electronic circuitry.

You can purchase TTL to RS232 converter boards on eBay.
They use the MAX3232 chip and you can purchase five of them for less than $2.
e.g. http://www.ebay.com/itm/5PCS-Mini-RS...-/131700206049

For 12V to 5V high efficiency, switched mode, voltage converters you can use the LM2596. Ten of them costs less than $4.
e.g. http://www.ebay.com/itm/LM2596-DC-DC...-/130729707683

I'm not affiliated with any of the above manufacturers. Paying up to $100 for a cable which costs less than $10 to manufacture is crazy.