There are plenty of SEO companies (or marketing companies) that will do one-time work on your site, but you have to look beyond the mainstream SEO services to find them. Look for people who perform link building, blogging, press releases, article marketing, etc, instead of "seo services".
The reason that a lot of SEO companies offer (or require) ongoing packages is because SEO is an ongoing task -- it's not a "set and forget" thing -- even in smaller niches. If you're dealing with a highly-competitive niche, then that requires even more attention as you constantly have to stay ahead of others who want your spot.
If it's a high-competition market, chances are the top sites are buying their links, or developing websites in that niche, getting them noticed by search engines, and then linking back to their primary site. A lot of expensive keyword markets utilize paid linking strategies -- they have to unless they're a special case as there is too much at stake. Web hosting, insurance anything, tooth implants, plastic surgery, etc...
If you're looking for guaranteed, fixed placement ("I need to be on the first page of Google", etc)-- you'll have more luck looking at some of the shadier SEO companies -- most ethical companies won't guarantee rankings as they can't.
You can try this search query:
http://www.google.com/search?hl=en&q=guaranteed+(%22top+10%22|%22first+page%22)+google+(ranking|placement)&btnG=Google+Search&aq=f
To see if you can find a company.
You might also want to try the digital point forums services section:
http://forums.digitalpoint.com/forumdisplay.php?f=60
You can find some SEO services in there, and while it's a gamble, you have real-user feedback to help you & it'll be cheaper than you'll find on a lot of websites.

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