One of the differences between Resident Evil 0 and Resident Evil is that in Resident Evil 0 there are no magic little boxes scattered throughout the game that magically transport all your items to any location you need them. In Resident Evil 0 if you run out of inventory slots your gonna have to leave a few items behind.
In Resident Evil 0 there is a handy little feature you can use to locate any item you choose. It's pretty, easy, and I'm pretty sure it mentions this in the instruction booklet (I haven't read it. Do people actually read them?). Head into your map by pressing Z or pressing Y and selecting Map. Then press A and a small bar will appear. This bar will show you all the items you've left lying around the level, and will point them out on the map so you can easily find them. I'm sure you already knew that though, as it's a very simple concept.
The one tip I'd like to make sure you remember, is that not all items will always appear on the map unless you make sure to include them. Just because you see a bunch of items in a room doesn't mean they will appear. The map will only show items that you have dropped. So all you have to do to make sure all the items show up is walk up to them, press A, and then select "no" when it asks if you want to keep the item. That item will now be marked on the map, so you can find it easier later if you need it.