Amazon has the Honeywell P4 Premium Portable Wireless Door Chime with Push Button for $16.66 with free shipping for PRIME members.
Wireless - Battery operated
Works with up to six (6) push buttons, motion detectors or door contacts for added security and convenience
Six (6) chime tunes, fully adjustable volume and six (6) visual alert icons
Self learning code system automatically adjusts to eliminate interference
450-ft operating range