Your family can buy and give you one.

Montana imposes no specific age restrictions on the purchase or possession of firearms. But Federal law restricts to age 21. You can get a Carry Permit at 18 and legally carry a handgun. (But you cannot buy one until 21). I know as you now do, this is one of the darnedest and stupidest thing I have heard.
Example, you can enlist in the military at 18 - they train you and put a fully automatic firearm in your hands - you perhaps fight for your Country and win the battle - afterwards you can't sit down to enjoy a beer (because you are not 21).

federal law prevents you from buying a Handgun from a FFL holder so no you can not order a gun from Buds as it would have to go to a FFL holder. Some states allow Private sales of Handguns to 18 year olds from the ATF site
(b) It shall be unlawful for any licensed
importer, licensed manufacturer, licensed
dealer, or licensed collector to sell or deliver—
(1) any firearm or ammunition to any
individual who the licensee knows or
has reasonable cause to believe is less
than eighteen years of age, and, if the
firearm or ammunition is other than a
shotgun or rifle, or ammunition for a
shotgun or rifle, to any individual who
the licensee knows or has reasonable
cause to believe is less than twenty-one
years of age;