Writers have drawn on different sources to create pseudonyms. Charles Lutwidge Dodgson created his pen name by translating his first two names into Latin--Carolus Lodovicus—and then anglicizing it into Lewis Carroll.
Samuel Clemens’s pen name Mark Twain came from his years working on Mississippi riverboats, where riverboatmen would cry “mark twain” to indicate they had arrived at a depth of water under the boat that made it safe to pass. He borrowed this moniker from Captain Isaiah Sellers; when Sellers died, Clemens started using the name. (Twain’s own stories have been called into question. Some biographers claim “mark twain” refers to a running bar tab he regularly drummed up when drinking at John Piper's saloon in Virginia City, Nevada.)
Daniel Handler came up with the name Lemony Snicket when he was researching his first novel, The Basic Eight, which is written under his real name. He had to contact right-wing political organizations and religious groups to have material mailed to him, but didn’t want to be on their mailing lists. So, when asked his name on the phone, he blurted out Lemony Snicket. It became a running joke with his friends and later, when it came time to choose a pen name for his children’s books, he knew he already had it.
You can find inspiration for a pen name anywhere—your real initials, a character name you’ve always liked but haven’t used, or geographical place names. Remember, sounds and syllables can convey a specific quality. The name Shoshanna Fells, for example, is softer than Brock Troust. Perhaps you’re looking for a more distinctive name or you want one that better suits your genre. Consider your motivation for using a pen name and let that guide your choices.