So I buy them off Amazon, but used to get them from hobby shops. If you just look up " model rocket engine" it'll have quite a few come up. You'll see engines ranging from a to e. A is the smallest, launching a few hundred feet. E is the largest, up to about 2,000 feet. They will come with different sizes, add if you get a kit, it'll tell you which size to get. To ignite, I use an Estes remote. Here's a link to one of them.
https://www.amazon.com/dp/B0006MZKG6/ref=cm_sw_r_apa_i_4Y.KEb3WMTXTW
Model rocket ignition system. 30' corded. Works well.
Edit: you will need ignitors as well.
http://www.amazon.com/Estes-302301-Model-Rocket-Igniters/dp/B0006NAQ64