Google Earth is free for personal use. You can find your latitude, longitude (not needed for barometer corrections), and ground elevation by entering the address or place name, then zooming in on detailed satellite photos and maps.
You can get your latitude coordinate and ground elevation from many local maps of your area. The most detailed is usually the US Geological Survey 7.5 minute, 1:24,000-scale quadrangle series topo maps, available from local outfitter stores, some bookstores, your local BLM or Forest Service office, and from the USGS Online Store. Some public libraries may have topo maps available.
To find your ground elevation, look at the wavy never-ending lines on either side of your location. They are lines of constant elevation called contour lines. Find some with numbers inserted in them. The number is the elevation of that line. In the United States they are mostly in feet, the contour interval between them may be 10 feet or more, and every fifth perhaps every 50‑foot contour line may be in bold. You may interpolate your ground elevation between the elevations of the contour lines on either side of your location.
Introduction to Topographic Maps, a tutorial from the GeoSpatial Training and Analysis Cooperative, offers detailed information on understanding and using topo maps.
Commercial GPS (Global Positioning System) devices have typical latitude errors of 10–15 meters which is good enough for the relatively small gravity correction, however the GPS typical elevation errors are too large for barometry where errors ideally should not exceed one foot.
To convert from degrees/minutes/seconds to decimal degrees:
(1) divide the seconds by 60
(2) add this decimal to the minutes
(3) divide this decimal minutes by 60
(4) add this decimal to the degrees
To determine your true altitude, refer to any of the above maps to find your ground elevation. To the ground elevation, add or subtract the vertical altitude of your barometer cistern's free surface above or below the ground level (or other precise level indicated on the topo map which can be measured locally).
Instructions, examples, and charts of barometer corrections for temperature, gravity, and elevation can be found in the Instruction Manual for 230-7410 and 230-7420 Series Mercury Barometers. (PDF 6.9MB)
A spreadsheet for automatic barometer corrections is provided by Princo Instruments, Inc. (Requires MS Excel or a compatible spreadsheet program.)