Hiya,
It really depends on what you want, personally I favor having on the site (option1) that way any SEO work you do helps build up the whole domain.
Here are some options to help you (only rough)
Option 1. - Create the product onsite
Pro's - helps build up whole domain, less work needed between two sites. Already existing site can help rankings.
cons - can clutter site, can backfire with any badly done SEO on the whole domain
option 2. - Create a sub domain (e..g sub-domain.example.com)
Pros - Can still help build you brand and gain association of site
Cons - link juice doesn't tend to flow between subdomains meaning more work
Option 3. - Completely separate domain
Pros - Can keep it separate from main domain (different link strategies etc.)
Cons - Absolutely no link juice flowed between it and main domain could double the effort to maintain two domains Fresh sites can be harder to rank
This is what I came up with in a couple of min and one else feel free to add to my pro's & con's as i'm sure i missed some bits and pieces.