Summa on Baptism of Desire with Comments
II-II Q100 A2 Reply to Objection 1: In a case of necessity anyone may baptize. And since nowise ought one to sin, if the priest be unwilling to baptize without being paid, one must act as though there were no priest available for the baptism. Hence the person who is in charge of the child can, in such a case, lawfully baptize it, or cause it to be baptized by anyone else. He could, however, lawfully buy the water from the priest, because it is merely a bodily element. But if it were an adult in danger of death that wished to be baptized, and the priest were unwilling to baptize him without being paid, he ought, if possible, to be baptized by someone else. And if he is unable to have recourse to another, he must by no means pay a price for Baptism, and should rather die without being baptized, because for him the baptism of desire would supply the lack of the sacrament.Saint Thomas is recommending omitting Baptism of Water, if a priest is only willing to sell it, because this would be cooperating in the sin of simony, the buying and selling of sacred things such as the Sacraments. One would reason that if Baptism of Water were absolutely necessary for salvation, one ought to be ready pay for it, even if this means cooperating in the sin of simony. The only other logical conclusion is that such a person must resign themselves to eternal damnation for they are left with an untenable situaiton. eitehr commit a mortal sin of simony to get baptized and go to hell, or go to hell for not being baptised. Surely a just and merciful God could not do this!III, Q68 A2 I answer that, The sacrament or Baptism may be wanting to someone in two ways. First, both in reality and in desire; as is the case with those who neither are baptized, nor wished to be baptized: which clearly indicates contempt of the sacrament, in regard to those who have the use of the free-will. Consequently those to whom Baptism is wanting thus, cannot obtain salvation: since neither sacramentally nor mentally are they incorporated in Christ, through Whom alone can salvation be obtained.Secondly, the sacrament of Baptism may be wanting to anyone in reality but not in desire: for instance, when a man wishes to be baptized, but by some ill-chance he is forestalled by death before receiving Baptism. And such a man can obtain salvation without being actually baptized, on account of his desire for Baptism, which desire is the outcome of "faith that worketh by charity," whereby God, Whose power is not tied to visible sacraments, sanctifies man inwardly. Hence Ambrose says of Valentinian, who died while yet a catechumen: "I lost him whom I was to regenerate: but he did not lose the grace he prayed for."Several Sacraments are necessary for salvation, Baptism and Holy Communion. Confession is also necessary for anyone unfortunate enough to commit mortal sin after Baptism. However he teaches that all three can be received in desire, if they cannot be received actually. 
Michael Fighting
