So this does require reading, but I want to make sure EVERYONE can get this achievement!
You'll thank me later!

What To Do:

1) (if you haven't already) to decompress zip files
(Don't worry if you don't know what a zip file is, just go here https://rarlab.com/download.htm and select the language and version suited to your PC)

2) (I did not make it) by using the "manual download" button on the middle of the page under files (save the download anywhere you want)

Download is safe, I've done this myself and can confirm.

3) Open the folder containing your download, and right click the compressed file and (which will decompress you file, making the map a usable file)

4) Next, find and which is typically located in:
Program Files (x86) > Steam > Steam Apps > Common > Half Life 2 > hl2 > maps

5) (achievement_lambdalocator.bsp) in the map location (from above, step 4)

6)
7) At the main menu, go to options > keyboard > advanced > Enable developer console, and make sure the box is checked and press "OK"

8) Press ~ to
9) (to load the map, try a couple times, easy to mispell)

10) When the map is loaded, or your assigned key to press it and congrats! Your achievement is unlocked! You're welcome!

Background Info

How did I find this?

Well, I'm lazy and there was no way I was getting this legit. I had recently played Half Life 2: Episode 2 and got the "Get Some Grub" achievement by downloading someone's map (from the Steam guides) but when I tried to use a Steam guide for this achievement, I noticed they all sucked. One just let's you discover how many are left, and the other requires you to modify "hex code?????" So therefore, I googled away for a game save and found this map online.
I'm an average guy who just wants achievements man..

It didn't work?

Please post a comment explaining your situation or send me a message and I will try to help you. If I cannot get back to you soon, please talk with others in the comments who have used the guide through the comments.

What else?

Lastly, I would appreciate if you guys left a comment saying it worked or not. I would like to legitimize this guide so that MANY MANY MANY players of Half Life 2 don't have to suffer through all of those lambda caches. Its amazing that after all these years, no one has made a Steam guide for this. I just want this guide to get as much exposure as possible so everyone can get that last achievement. Feel free to browse my achievements for proof that I have them all! Hope this helps!