As with most other seasonable durables, the best way to get into Halloween costumes on a small scale is to scour your local sellers, as well as online retailers, in the days after Halloween. Store the stuff for eleven months and sell it when the season rolls back around. I used to do this every year with every single holiday, and here in the US we have a major "themed" holiday almost every month (including, thankfully, no less than three "patriotic" holidays where the decorations are all pretty much the same, so you can sell the same stuff three times a year if you have it.) Selection will be limited, but not as bad as you might think; if you can swing the storage space I believe you'll find it pays to try it.
"Failure is not when you fall down. Failure is when you don't get back up."