A few sites on the Red Heifer:
Kindgdom Bible Studies Ashes of a Red Heifer Part 1
ASHES OF THE RED HEIFER
...............Numbers 19 is a most unusual chapter in the Old Testament. All the offerings in the Bible are bullocks and rams, but here there is an exception - a heifer, a female cow whose never given birth. All the offerings in the Old Testament are slaughtered and offered to God, but this offering of the red heifer, though killed and burned, is very different from the rest. While all others are offered to God to meet current claims - that is, the sin-offering, the burnt offering, or the peace offering according to the need of the day - the red heifer alone was not for the present need.
It was offered to meet future needs. The ordinance of the red heifer stands alone. While other sacrifices are often brought before us, this recorded in no other part of Israel's history...........
Studies In The Scriptures - Tabernacle Shadows - Chapter 7
"THE ASHES OF A HEIFER"
......ONE feature of the ceremonial law of Israel, related in `Numbers 19`, required the killing of a red heifer (cow)--one without blemish and which had never been under the yoke of service. It was not one of the sin-offerings of the Day of Atonement, nor was it one of the offerings of the people subsequent to the Day of Atonement--indeed, it was no "offering" at all, for no part of it was offered on the Lord's altar or eaten by the priests. It was sacrificed, but not in the same sense, nor in the same place, as these offerings--in the Court. It was not even killed by one of the priests, nor was its blood taken into the Holy and Most Holy.
The Red Heifer was taken outside the camp of Israel, and was there killed and burned to ashes--flesh, fat, hide, blood, etc.--except a little of the blood taken by the priest and sprinkled seven times toward the front of the Tabernacle (Revised Version and Leeser). The ashes of the heifer were not brought into the Holy place, but were left outside the Camp, gathered together in a heap, and apparently accessible to any of the people who had use for them. Under the prescription of the Law, a portion of the ashes was to be mixed with water in a vessel, and a bunch of hyssop dipped into this mixture was to be used in sprinkling the person, clothing, tent, etc., of the legally unclean, for their purification.