Everything depends on what you are planning to put in the bags. If it's something heavy, you may need to invest in canvas totes as your basic party bags would prove to be too flimsy. You can find basic favor/treat/goodie bags at any place like PartyCity or even on Amazon.
What do you plan on putting in these bags? Once you can answer that question, it will be easier to find a bag that will hold the items. Wouldn't want to suggest a place that carries light/ thin bags if you're packing something on the heavier side.
Re: welcome bags
Mine was from Dollar tree.