I do not want to go to Apple to upgrade the SSD on my macbook pro 12,1. Is there a third-party SSD that would work with my Macbook Pro? I currently have the 128GB model, and it is running out of space. I am looking to purchase either a 256GB or 512GB SSD that will run at comparable speeds to the current SSD. I know Apple has tried to stop users from upgrading to third party SSDs, I am more than comfortable upgrading these things by myself, but I just don't know if there is any compatibility. Thanks.
Have a look here at MCE USA. They offer SSD Blades for MacBook Pro Late 2013 - Early 2015 from 256GB up to 1TB. http://store.mcetech.com/mm/merchant.mvc?Store_Code=MTOS&Screen=CTGY&Category_Code=PCIEFLASH
OWC/MacSales doesn't have any yet.
It seems like OWC has finally managed to build replacement SSDs for the new macbook pro retina models, including the 13" early 2015 model (480 gb model): http://www.owcshop.eu/catalog/product_info.php/aura-macbook-retina-mid2013-till-2015-480gb-preord-p-1380#.Vup7ExIrK-Y
Your MacBook may be a bit too new to get an upgrade for, however THIS site provides SSD replacement drives for MacBooks made between 2008 - 2013, depending on the model.
You might contact the company to see if your 2015 MacBook can use any of the drives they sell.
Also, these drives are very highly rated on Amazon.com, so I think you can feel safe that these are a realistic option.
Transcend also has an SSD upgrade for this model. The Transcend JetDrive 820. In Europe, it's easier to buy than the OWC brand already mentioned in another answer.
I do not want to go to Apple to upgrade the SSD on my macbook pro 12,1.
That's actually not possible, AFAIK they don't upgrade SDD on existing MacBooks
You can buy 3rd party SSD from Transcend or OWC
For your model the supported drives are: JetDrive 820, 825, 850, 855
OWC: Aura and Aura Pro X