Big thumbs up for Other World Computing (www.owc.com). I bought the kit that mounted a SSD along with the 500 GB existing hard drive. Make sure you have a good Time Capaule back up. Then with this kit and a $100 SSD you can add a 256 GB SSD and make a 750 GB Fusion Drive.
OWC has a step by step color book tutorial that comes with the kit that takes a truly formidable task and makes it easy. Just find a clean table with good lighting and take your time. The result is amazing. I go from off to up and running in 23 seconds from chime to operational screen.
Ditto OWC. I have bought upgrades from them numerous times before retiring.