Check out Billythebrick Cosplay! He's a Cosmaker in the PNW who does costume and prop design and fabrication works
If somebody wants a full costume for Halloween, it may already be too late! It really depends on how elaborate it is. As far as props or items in his shop though, about 2 weeks out for placing an order to guarantee something arrives before the holiday.
As a general rule, it's more practical for client and creator if you order at least 2 months in advance. One of the most irritating things for an artist is being given more to do than they have time to achieve. Also, never offer "Exposure" as a compensation. They'd get it either way, so what you're really saying is work for free, when it's not free for them to make the craft in the first place.