I have decided to try and mount a light on my AR.  I have a CMMG Midlength 16" carbine.  It has the standard, traditional AR plastic handguards.  

Do I need to buy railed handguards or is there a more thrifty way to securely mount a light on my weapon?

You can buy a section of rail to add to your factory handguard. Any of the suppliers will have them for pretty cheap.

You can also buy rails that attach to the bayonet lug or bolt into the front sight.

Thank you.

I bought an  Add on Rail  from Midwest Industries and bolted it to my standard USGI Handguards.  I have decided that I want to put a Vertical Foregrip there and have the light on the side.
So I bought GG&G SliC Thing  and their One inch Light Mount which will allow the light to be mounted on the front sight and move the sling attatchment to the side.  I thought about the elzetta one but didn't want the light underneath.

(BTW, Prior to this I used the Vikings offset light mount but did not like it at all.