offline yosemite 10.10.2 installer

There is no offline version of the full installer. It is only available at the app store.

There is an update offline version - I found the combo update 10.10.2, but that requires having 10.10 installed:

OS X Yosemite 10.10.2 Combo Update

If/when I download a full installer, I install it and then make what I call a "factory fresh" clone of my entire system on an external hard drive. That is my base/clean install clone and is not updated purposely for the times I may need that. I also regular clone my system on another partition to have an up-to-update clone as well.

No. There is no offline standalone installer. But you can make one if you have an 8 GB or larger flash drive partitioned and formatted for OS X. You will need to download the installer application (not an update installer.) This is the one with the 5.3 GB file size.

Make Your Own Mavericks, Mountain/Lion Installer

After downloading the installer you must first save the Install Mac OS X application. After the installer downloads DO NOT click on the Install button. Go to your Applications folder and make a copy of the installer. Move the copy into your Downloads folder. Now you can click on the Install button. You must do this because the installer deletes itself automatically when it finishes installing.

2. Get a USB flash drive that is at least 8 GBs. Prep this flash drive as follows:

1. Open Disk Utility in your Utilities folder.
2. After DU loads select your flash drive (this is the entry with the mfgr.'s ID and size) from the leftside list. Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Set the format type to Mac OS Extended (Journaled.) Click on the Options button, set the partition scheme to GUID then click on the OK button. Click on the Partition button and wait until the process has completed.
3. Select the volume you just created (this is the sub-entry under the drive entry) from the left side list.
4. Click on the Erase tab in the DU main window.
5. Set the format type to Mac OS Extended (Journaled.) Click on the Options button, check the button for Zero Data and click on OK to return to the Erase window.
6. Click on the Erase button. The format process can take up to an hour depending upon the flash drive size.

Make your own Yosemite flash drive installer using the Yosemite tool:

You can create a Yosemite flash drive installer via the Terminal. Yosemite has its own built-in installer maker you use via the Terminal:

You will need a freshly partitioned and formatted USB flash drive with at least 8GBs. Leave the name of the flash drive at the system default, "Untitled." Do not change this name. Wait for the process to complete which will take quite some time.

Open the Terminal in the Utilities folder. Copy and paste the following command line in its entirety into the Terminal window.

sudo /Applications/Install\ OS\ X\ Yosemite\ Developer\ Preview.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ Yosemite\ Developer\ Preview.app --nointeraction

Press RETURN, enter admin password (will not echo to the window) then press RETURN again.

You need to have the installer in your Applications folder or change the paths in the above command line.

Creating a bootable OS X installer

If you can wait for the next release of OS X, you may be able to use the method in the Apple Support article.