Just stating: Even if you were to get Rei's Minimap for 1.8, you wouldn't be able to use it since it'd take Forge. Forge currently has a bug in it where you cannot connect to 1.8 Bungie servers [aka what we're using] and it'll say something along the lines of "Could not connect to Bungie [World] Server. Please make sure you port are port forwarding properly" or something like that.
Instead, I'll give you a list of what you can try instead.
This written guide will contain information concerning something called Liteloader and Voxelmap [Formally Zan's Minimap].
Liteloader Standalone
First things first, we'll need Liteloader. Conveniently you can download it through
here.
Secondly, you'll need VoxelMap. Conveniently placed link
here.
1. Run the Liteloader Installer (.exe) file that was downloaded. It will ask you on how to install it. Make sure that "Extend from" is set to 1.8, and the profile name is up to you. From there, just click OK.
2. Go to your .minecraft folder [Generally located in %appdata%] and go into said .minecraft folder. If there isn't one already, create a folder named "mods".
3. Download and place the Voxelmap mod into the new "mods" folder we created.
Liteloader + Optifine
This version is almost identical. BUT! It has a crucial beginning portion otherwise it will not work!
1. Download Optifine and install it to a blank 1.8 [STRICTLY 1.8] profile.
2. Download Liteloader installer (.exe) file that was downloaded. It will ask you on how to install it. This time, we're going to click the drop down for 1.8, and select the Optifine profile that was just installed. and the profile name is up to you. From there, just click OK.
3. Again, navigate to your .minecraft folder [Generally located in %appdata%] and go into said .minecraft folder. If there isn't one already, create a folder named "mods".
3. Download and place the Voxelmap mod into the new "mods" folder we created.