I bump up against this problem myself all the time and it's so infuriating (doubly so when most of the domains you want are registered but not even being used).
Finding a domain name that is available, memorable, easy to spell, snappy AND which quickly gets the idea behind your product/service across is HARD but my advice is to persevere with it. A good domain name makes a big difference when it comes to user traction and seo (and, for me, just feels better than inventing some random name that bears no relation to the site).
A lot of the time I find the .com is taken (but not being used) and the .net is free. So I register the .net and then try and pounce on the .com when it comes up for renewal (this is an art in itself). If the .com gets renewed I occasionally (depending on how badly I want it) approach the owner and ask if they'll sell it (be warned some people have ridiculous ideas of how much a domain is worth).
An alternative is to try extensions other than .com (examples: .io .in .is .to). This is what pinboard.in did and I think it works (pinboard is easy to spell, memorable, snappy and does a good job of explaining what the site does). I use www.eurodns.com when searching for weird domain extensions (as you can check virtually all of them in one go).
Failing all of that, get a thesaurus, dictionary and a few bottles of wine and just lock yourself in a room until something clicks!
