I have an excel worksheet filled with items and data about them. Each
row has one item plus various fields of data about that item. Now,
these items get data from different places, and I need to merge that
data. Here's the problem.

Say that my worksheet contains hundreds of rows like this:

item1 stat1 stat2 stat3
item2 stat1 stat2 stat3
....
item999 stat1 stat2 stat3

Now say that stat3 comes from a webpage with the items listed like this:

item12 stat3
item23 stat3

Because the webpage contains stats for some, but not all, of the items
in my list, I can't just copy and paste the table into my list in
excel: it won't match up.

So I need a suggestion for the best way to get this data into excel
and matched up. This is something I will have to do on a regular basis
from a number of website, each of which provides stats for only a
subset of the items.

This is called "web scraping" and there are many utilities that do
this if you don't want to roll your own:
://www.google.ca/search?hl=en&sa=X&oi=spell&resnum=0&ct=result&cd=1&q=web+scraping&spell=1

(Not a researcher so this isn't a proposed answer)