I have a couple devices I took apart and they have a lot of these chips (rfp40n10) inside them. Here is a link to the datasheet:

I tried to find these for sale but both digikey and mouser didn't have a price listed. I'm wondering if there is an alternative in case I need to buy more or replace them if I use them in a project build.

 

Any other mosfet in the same package (TO-220AB), 40A or higher, 100V or higher, 0.040 ohm or lower should be an acceptable replacement. From the datasheet they are 16 years old, so you should not have trouble finding a suitable replacement, just check that the Vgs(th) parameter is in a similar range. The Qg gate charge might need to be similar or lower as well, but that really depends on in what circuit they are used in.