This should be straightforward if you have the right tools already. The key things you need are good RAID management software tools, and a good cloning software package.
How are you running the RAID array? If it is through the RAID controls built into your mobo and its BIOS, you probably have what you need. If it uses an add-in RAID controller card in your PCI slots, I would expect you have or can get the RAID tools needed. If you are using the software RAID system included with Windows, I don't know what tools they provide but they should be available. In all cases, the secret is to find the instruction manual for the RAID controller and its software tools. If you're using the built-in mobo RAID system the instructions may be included in the mobo manual, or they may be a separate manual you can download. Go to either the mobo manufacturer's website, or the website of the maker of the main chipset on your mobo to find it.
You also need cloning software. Acronis True Image is really good and maybe you can install it on your RAID machine. If not, you can get a free customized version of it by download from Seagate (they call it Disk Wizard) or WD (Acronis True Image WD Edition). HOWEVER, each of these free packages will make a clone ONLY TO a hard made by the supplier. So you can use Disk Wizard ONLY to make a clone TO a drive made by Seagate - they don't care which old drive you are copying FROM. So, wherever you buy your two new drives from, look for their free cloning software download, too, and its instruction manual.
The overall sequence I suggest should really be done in your machine that has the RAID drives in it. Moving them to another machine could just add complications. The sequence depends on the character of RAID1 arrays - you have two identical copies of the same thing on the two members of the array.
1. Download and install the cloning software package on the RAID machine.
2. Use the RAID management software tools to break the array apart into two separate drives. After this step you have two identical individual drives, each with usable copies of the original RAID data. Shut down the machine, remove one of those drives to keep externally as a good and complete backup so far. Now mount the two new drives in the machine, configure as SEPARATE drives but you do NOT need to Partition or Format them because these steps will be done for you anyway. Boot the machine from the old single drive that used to be half of the RAID1 array.
3. Run the cloning software to clone both Partitions on the old drive to ONE of the ones. In doing so, pay attention to Partition sizes. By default the software may offer to create the two Partitions on the new drive in PROPORTION to their sizes on the old one, and that may not be what you want. If your new drive is five times bigger than the old one, I doubt you need your System Partition to be five times bigger. So manually set the Partition sizes, with the new System Partition not much bigger that the old one, and the new DATA Partition all the rest of the new drive's space. Of course, you will have it make the new System Partition bootable.
5. For the next couple of steps I'm going to write as if you were using built-in RAID controllers on the mobo. If that is not the case, adapt the concepts and use the correct procedures according to the instructions for your system. When the cloning is done, shut down. Remove the remaining old drive unit, perhaps reconnect SATA data cables so the new drives are using the same mobo pinout connectors as the original RAID drives were. Reboot and go directly into BIOS Setup to ensure that the two new drives are configured correctly, but still do NOT configure them as RAID drives. Ensure the one you have prepared so far (with its clone copies) is the boot drive. Save and Exit and let ti boot up. Just make sure everything is working nicely, but you still will NOT be able to see the second new drive - it has not been prepared.
6. Reboot and go directly into BIOS Setup and configure the two drives as RAID units in the SATA port configuration. Save and exit. As the POST screens come up there will be a prompt to press particular key(s) to enter the RAID setup screens. Do that. Choose the procedure to Create a RAID1 array by Adding a second drive to an existing single drive full of data. The system will take care of Partitioning and Formatting the second drive and copying all data over to create the mirror image of the first drive on the second. When it is done I expect it will reboot your machine and it should come up with the entire process complete and your new RAID1 array working exactly as you want.
If this all works as planned, you have two separate old drives on the desk, each with perfect copies of the old system that could be used as backups to run from if anything goes wrong. But if it's all good, you can decide what to do with those units later.