Originally Posted by kows
the easiest way would be to load all of your items and use javascript to display only the items you need to given the category. I can't really help you much with the javascript stuff in that situation.
alternatively, and probably less effective/useful, you could have multiple pages for your form, and once a person selects a category, they would press some button titled "next" and then you would select the category you would want. this would require the user to press back, though, to change the category at the last minute.. which is sometimes not desirable.