I'd need to know more about your site hosting package and your web authoring software to best advise.
FrontPage has a built in site search feature using an extension.
It would help a lot to know the type of hosting account you have, whether it features PHP/MySQL support, ASP support or if you are allowed to run custom CGI scripts.
For a truly useful and usable product search form you will have to get into a database of some description at some point. Keeping an online database of your products, (name, type, category, number in stock, cost per item etc.,) allows you to query that database using one or more forms and your clients can find out what you have and how much it costs, product specification and whatnot.
This is right about where most people call in a professional web application developer though. It may not be rocket science but unless you already have a reasonable knowledge of database design and implementation coupled with some indepth understanding of web scripting language(s) and syntax, you're likely to become horribly bogged down.
Happily for you there are some kind souls out there who offer their code for your own use and you install and configure it on your web.
If you let us know whether your account has any dynamic content support (as mentioned above) one of us will advise further.