The wedding is located in Boulder, CO. You can use websites like Expedia, Travelocity, Priceline, etc. to find good deals in April. Surrounding areas, other than Boulder, if you don't mind driving a bit further include:
-Broomfield
-Louisville
-Lafayette
Flight
Denver International Airport
DEN is the closest and most convenient airport to fly into for this location.
Travel Note
Uber/Lyft
The venue is only about three miles into Boulder canyon so there is easy Uber/Lyft access to and from if needed!