Fiona,
I have been a professional architectural compositor/retoucher for 7 years. I can tell you that there are not many dedicated arch retouchers out there. It's all I do 10+ hours a day in
PS. I am one of only a few here in the states that I know of. Most are freelance retouchers covering a variety of different forms of photography (i.e. fashion, advertising, fineart, arch etc). I think the reason maybe the somewhat unpredicable nature of architectural photography. I would hit every forum and job board you can.
You might have to settle for someone whos expertise lies in a different area but is willing to do dedicated arch retouching. You want someone who has a famialiarity(sic) or experience in architecture first off. It helps if they like the work. Next would be excellent masking/compositing skills, and reconstruction retouching with good knowledge of perspective control. Color correction with knowledge of how to control local contrast and luminance ranges of parallel and offest surfaces as well as setting the mood of a space overall through these techniques. Most retouching can be daunting but architecture poses unique problems sometimes and architects and designers can be quite nitpicky.
You might want to put up an ad in New York and Chicago as there is a great number of Architectural firms and Arch photographers in these areas. Retouchers like cattle follow the work. I hail from Chicago. Most of our clients are Chicago based arch firms, construction companies and designers but we have national clientel as well. There is a lot of work here. Incidentally, depending on experience highend retouchers can fetch pretty good coin. Anywhere between $35,000 to 60,000 USD and higher. In my case I fall into the latter range with 10 years experience in digital production, 7 years of pro arch
PS work.
Good luck with your search.