Err,
You can purchase guns over the 'net, but they have to be shipped to an FFL in your area (who will add on his own fee to play mule for you. You can shop for an FFL who will charge the least.)
Second, there are many ranges/shops across the US that will rent guns before you buy, but not knowing where you live, I can't really help you.
You might go on
www.ar15.com and ask them. (registration is free and easy, like TFP)
God knows someone there will be able to help you in your area of the country.