There are three ways to get information about previous products. One is a patent search, which shows all the similar product ideas that have been patented. This was Christini's initial research point; his patent search at a public library in Philadelphia uncovered six or seven recent patents for all-wheel-drive mountain bikes. (You can do your own Internet patent search at http://www.uspto.gov.)
Because Christini's invention is a mechanical device, most, if not all, competitive products are likely to be patented. Product categories that involve less engineering--say, a clip to hold a seat belt away from the driver's neck--usually have a lot of products on the market that aren't patented. If your product falls into this category, your best bet is either to call the sponsors of major trade shows in your industry and ask for old show directories, or to look through new product announcements in back issues of trade magazines.
In Christini's case, the major trade show is Interbike, and a key trade magazine is Bicycle Retailer and Industry News. You can find trade shows for your industry in any number of trade show directories, available at most larger libraries, where you'll also find Gale's Source of Publications, a listing of trade magazine names and addresses.