Where did the high priest get the coals he carried into the inner altar?
The high priest took the coals from off the brazen altar. On the Day of Atonement especially, he carried those coals into the inner altar, the small golden altar of incense, where he burned incense rather than animal fat. The smoke covered the room, symbolizing Christ's finished work.
Les explains that the high priest would take coals from off the brazen altar and carry them into the little altar inside. This was done especially on the Day of Atonement, though Les says it was likely also during the year. On that inner altar he would not burn animal fat; he would burn incense. The smoke of that incense would literally cover the little room and, as it were, obliterate the very presence of God from even the high priest. Les also notes that the altar of incense, according to the book of Revelation, represents the prayers of the saints, so believers are involved in that as well.