I can only answer for ordinary town houses in Norway, but the technique was imported from Germany and definitely similar in Britain.

Gargoyles, decorative bands and other stone ornaments on 19th century neoclassisist or gothic buildings were not commissioned, but ordered from catalogues. (Brochmann, Odd: "Bygget i Norge: en arkitekturhistorisk beretning" Oslo, 1981. Vol. 2, page 150.) (Some such catalogs are kept at the National Library of Norway, unfortunately I don't have the exact source as they are not catalogued individually (since they were not published per se) but kept in thematic boxes the small prints collection.)

Anniken Thue at the West Norway Museum of Deocrative Arts mentions some background for this development:

As industrial production became prevalent, a broader transport network made for a vastly increased variety of available products. The technological advances soon affected the architechture as well, for instance the Chrystal Palace was an achievment for the producers of prefabricated glass panes and wrought iron modules. Cast cement decorations became obiquitous, and and catalogues became indispensable to architechts and builders.
(Thue, Anniken: "Plysj, palmer og pomponger: en hyllest til historismens epoke 1840-1900". Bergen, 1987. Page 28.)

These decorations were sculpted once in plaster, and then cast in cement in the quantity needed as orders came in. Short answer, most were mass produced by anonymous sculptors who probably also designed other plaster objects like ceiling roses, wall strips and knick-knacks. In the case of Norway, I would assume some plaster originals were bought from larger factories in Germany or Britain. However, the catalogues I have seen do not mention who produced the originals, only what local company recast and sold them.

For most buildings, it would be the owner's decision whether to pick from the catalogue or leave that to the contractor. According to Brochmann (cited above), many architehcts' blueprints left a blank space for the decorations to accomodate the owner's taste.

I am sure there are cases of wealthy people (or churches, etc) working more closely with the architecht to commission specific motifs or materials. In those cases, I don't know what the process of hiring a sculptor was, so I'll leave it at that: the vast majority of such gargoyles were mass-produced catalogue items.