There's isn't a specific place (like webmasterworld.com for webmasters). Start one. But you may find it difficult getting the professional PC building companies to sign up to a forum - you may end up with a lot of the average or garden enthusiasts who build PCs as a hobby.
To reach them via advertising you could use specialist trade magazines like CTO and others (in the UK). And trade directories like VNU's. There may be a way you can partner with the big distributors like Computer2000 to avail of their customer database. (They won't rent you the list but they could send an email out on your behalf if they take on your products). There are also associations and professional bodies whom you may be able to persuade to let you speak at a conference. Post or sticky me the product type and I may have other ideas.
Already in with major distis but would like to reach the small-medium sized builders and resellers to generate some pull in the US. Experience tells me the co-op ad programs with distis are often overpriced and poorly executed.