Most of the "search engine" sounds are terrible, 11 kHz, 8 bit wave files from the 1980's era of Sound Blaster cards. Stay away!
I find that I can sculpt synthetic sound effects using the Nyquist command prompt that is built into Audacity. That approach does require an understanding of DSP, though. For life-like sounds, I use purchased sample CDs. I purchased a bunch back in the '90s for music making, but some of them come with a decent set of sound effects, too, which can often be warped into service.
Here's a Nyquist script for an explosive "boom" if you want to try it:
; boom
(mult (reson (scale 0.17 (noise 1.0)) (pwl 0 500 1 100) (pwl 0 400 1 40)) (pwl 0 1 0.1 0.4 0.2 0.2 0.3 0.1 0.4 0.05 0.5 0.025 0.6 0.0125 1))