Before you throw down big bucks for us to ship a bag of this fine-grain silica sand to you, check with your local Masonry &/or Landscape Supply as this is a commonly stocked item.  Depending on your location, prices can range from $10 - $15 per 100lb bag of silica sand.

Another way to find fine-grain sand - contact your local Pool Supply Store.  Pool filter sand is perfect fine-grain silica sand! It is typically sold in 50 - 60lb bags and costs about $12.00 a bag.

High-Temperature Mortar Mix Recipe: 

3 Parts Silica Sand
1 Part Quicklime
1 Part Fire Clay
1 Part Portland Cement

How to make High-Temperature Mortar