I've been looking around forever for an aftermarket cruise contol kit for my non-egas car. A friend of mine put an Audiovox CCS-100 kit on his K-bike a few of years ago and says it works perfectly, but I found that it was discontinued when I tried to buy one.
This company, Murph's Kits, is now importing a clone version made by the company that originally made it for AudioVox and are offering them for sale. It cost me about $200 to get it to my door, and I think it's going to be a lot cheaper and easier than trying to retro-fit factory cruise to a car that didn't come with it.
As far as I know, it is the ONLY aftermarket kit available that will work on a 97 - 99 Boxster. Here's the link:
http://www.murphskits.com/catalog/product_info.php?products_id=422