There are many options available, but the cheapest and easiest option at your disposal is toy guns. There are plenty of toy guns available to kids these days, which with a quick make-over, can be turned into mean-looking movie weapons at very little cost.
Water pistols are always a good bet. Don’t be put off by the fact that they come in fluorescent colours either – nothing that a can of matt black spray paint can’t fix. Water pistols sprayed matt black turn into surprisingly realistic-looking guns (great for most scenes other than close-ups). To help your actors with realism, fill the water pistols will sand to give them a little weight.
Your other options are of course things like cap-guns, starter pistols, and replicas (if you can get your hands on them). Remember the matt black spray paint if they need added realism.
There are a few stores online which sell useful items. Swords N Stuff sells what its name suggests. Tuscany Trading has a bunch of replica guns and swords, 411 Toys also stocks plastic guns, and The Costume Page has a great list of places to get accessories. For those in the UK, try Firesupport.
One good ideal for guns for movies are Airsoft guns. For those of you who don’t know, Airsoft guns are 1:1 replicas of real firearms, only they shoot 6mm plastic BBs. Most even use realistic action motions, so when you shoot your Beretta the slide moves back and returns for the next shot just like a real one. Try Shorty USA, or in the UK, Airsoft Dynamics or BB Guns.
As a final thought, remember that the safety of your cast and crew is paramount at all times! Even cap guns and blanks can cause injury and even death, and in many countries if someone dies on your set, the director can even be charged with man slaughter! More information on safe use of firearms on set can be found at the web site of Australian armourer John Fox.