What is the case screw length and thread pitch for the 2010 MacBook 13" Model 7,1?
You didn't mention which screw you lost.  It's important to know because there are a number of different screws specific to that model:


3 mm Phillips #0. (Head: 2.75 mm. dia.) 
4 mm Phillips #0.  (Head: 2.75mm dia.)
6.25 mm Phillips #000. (Head: 4 mm. dia. x .5mm thick)
3.25 mm Phillips #000. (Head: 4 mm. dia. x 4mm thick) 
11 mm Phillips #00, with Shank (2.2mm dia. x 2 mm len.) (Head: 3.2 mm. dia. x .5mm thick) 
.25 mm Phillips #00, with Shank (2mm dia. x 3.75 mm len.) (Head: 3.2 mm. dia. x .5mm thick)
5.2 mm Phillips #00, with Shank (2.3mm dia. x 3.5 mm len.) (Head: 3.2 mm. dia. x .5mm thick) 
11 mm Phillips#00 in the middle of the case. (Head: 5mm dia. x .75mm thick)
14.5 mm Phillips #00 (Head: 5mm dia. x .75mm thick)


Source:  ifixit.com

Your best bet is to browse through the repair guides on iFixit, identify the screw that you are missing then search for it online.  However, if it's just one screw, you will probably be just fine.