Agreed with maflynn above.
If you wanted an SSD in the iMac, you should have BOUGHT the iMac with the SSD already installed.
Having said that, if it's an iMac with a platter-based HDD inside, I'm wondering if it has the necessary connector for a PCI-e based blade SSD "in place"? (others will have to answer that)
If you really want SSD speeds,
here's the cheapest, easiest, fastest, and SAFEST way to do so:
Get an external USB3 SSD, plug it in, and set it up to be your "external booter".
There's next-to-nothing involved in doing this, particularly if you buy something like this:
http://www.amazon.com/exec/obidos/ASIN/B00ZTRY532?tag=delt-20
Just velcro it to the back of the iMac's stand, where it will be out-of-the-way and all-but invisible.
An externally-mounted SSD in a USB3 enclosure will run at about 80-85% of the speed of an internally-mounted SATA SSD.
Yes, it's a little slower, but hardly noticeable (if noticeable at all) in normal day-to-day operations.
Again, do it this way, and you'll be amazed at how easy it can be.