Hi Zen,
You should be able to get horticultural grade sand that is prewashed and screened from a builders merchants/DIY/Home improvements store. You can get like 20KG for like $2 or something. Get the larger grained stuff 3mm or above would be best.
You'd probably spend close to $2 boiling enough kettles to sterilize a decent quantity lol. Not only that, sterilizing with hot water may kill most/all the bacteria in it, but does not remove any tiny seeds from weeds etc. In the long run, it would be more messing around than its worth probably.
Live dangerously... buy a bag
All the best,
Aaron