Well, yes, it is possible to buy ammo on line. Check ammo man, cheaper than dirt, midway, shooters supply, or just run a google search for ammunition sale. There is also ammo for sale on the two largest auction sites, gunbroker.com, and auctionarms.com.
Will it save you money? A resounding "maybe". Price of ALL ammo has jumped- as the availability of ammo has slumped. The best bargains out there are still military surplus rifle ammo (8mm Mauser, 7.62 Russian, etc) However, when it is in stock, you may save a few bucks over local store. DO remember that you will pay shipping, and ammo is heavy.
.40 and .380 seem to be among the scarcer ammo right now. Would be a great time to give some thought to adding reloading to your hobby list.