* er: This is the most common spelling, like in "her, bird, term, worker."
* ir: Like in "bird, girl, first, stir."
* ur: Like in "fur, church, turn, burn."
* or: Sometimes "or" makes the "er" sound, like in "word, work, worm, world."
To give you a better list, I need a bit more information:
* Do you want words with the "er" sound spelled specifically as "er" ?
* Are you interested in words with all the different spellings of the "er" sound?
* Do you have a specific length or type of word in mind?
Let me know, and I can give you a much more helpful list!