To find all the files of a certain extension within the current path you can use the following find syntax. It works by making use of bash's built-in glob construct to match all the names having the .extension.

find /directory/to/search -maxdepth 1 -type f -name "*.extension"

To find all files of type .txt from the current directory alone, do

find . -maxdepth 1 -type f -name "*.txt"