Course development: can I pay someone to make slides for the course?
Theoretically you can, but practically I doubt you will find someone qualified who is willing to do it for a reasonable price. Designing a course is hard work and pretty individualized. Look at how many different ways there are to teach the same course. Everyone does it differently and I hate teaching to someone else's syllabus and cannot image using someone else's unaltered slides.
As for acknowledging the author, you probably need to let your department know that you have outsourced this aspect of your job. You will need to make sure that the material is appropriately licensed so that you and the department can use it as needed.
As for telling students, I believe there is a fair amount of leeway regarding the reuse of material. Specifically, for many courses, you are not presenting "original ideas" so it is not plagiarism in my opinion. That said, when possible, redirecting students to the original source can be helpful.
In my disciplines of math and computing, most of the undergraduate textbooks come with instructor resources including prepackaged lecture slides, to assist with exactly this concern. I find that the quality ranges from bad to barely-adequate. About half the time I can use them as a starting point (and otherwise need to restart from scratch). So unless you are developing a very nonstandard course, you could research textbook options and pick one that comes with lecture slides as a start. I would expect to be editing, refining, and customizing those slides for future semesters as you learn more about teaching the course.
In effect, it sounds like you want to sub-contract course design, or at least part of it. In theory, I don't see an ethical issue with this as long as nothing happens that will disadvantage the students taking the resulting course. The important issue is that the students get a high quality course, whether delivered by you or someone else or developed by you or someone else.
Of course it will be your responsibility to judge accurately whether the resulting course meets appropriate standards and to guarantee that it does when delivered. You are, in effect, taking on a management role in course development and are completely responsible for the result. But the process is less important than the result, IMO.
It will also be your responsibility to deliver the course in an appropriate (say, flexible) way. Students, as usual, need reinforcement and feedback no matter how the materials were developed.
I will note, I hope accurately, that courses delivered by Open University UK are (a) high quality and (b) developed by large teams, including, in many cases, the production facilities of BBC. The process is very involved and takes quite a bit of time, but the results are high quality. I suspect that other online courses have similar team development structures.
Normally it is good to acknowledge such help, but that can be a contractual issue and may need to be if it seems unwise to have students contacting the developer for any reason.
Sorry, but I can't help in finding such a developer.