I hope this isn't too late to be helpful to you. I haven't done the SCJP, but I have 10 Sun sys admin, storage, and security certifications. In 8 cases I did not take any of the recommended courses, in two cases I took 1 of 2 recommended courses. Like you, I was working in the field. I have always used the same study strategy. I take Sun's listing of what concepts are on the test, figure out which ones I already know well, and then find documentation from Sun websites (and books if I have to) related to the topics I'm weak on and study from those. Also, sometimes the Sun exams include information that I would think of as more "marketing" than technical. Skiming the marketing product pages for any products that are related to the exam (ie J2SE, etc) can get you an extra correct question or two.
Best of Luck,