You will not find a quality marketing professional to work on a pure commission basis, nor will you find a sales person to work on a pure commission basis without any marketing support.
Even if the marketing professional will put their own compensation at risk, there are hard costs associated with marketing -- printing, advertising, licenses, admission fees -- that cost real cash.
A marketing budget is as important to your overall business as the right tools, and knowing what to do when you log into the back end of your customer's server.
Of course there are lots of ways to do it yourself-- but they will all involve considerable investment of time. During the time you are being your own marketing professional, you will not be earning revenue. The result is that it will always get postponed for direct sale/production. The result is it probably wont get done, or will only be done in "waves" when you don't have work.
Is it worth Investing In?
At a defining moment you will decide if your computer repair endeavor is a hobby or a business. When you decide it is a business you will make the hard financial choices to invest the money that is necessary to support the business growth your require.
What are the numbers?
I know the numbers are right on for you -- but play for a second with this model:
Lets say that you currently charge $50/hours. And you are billing at 60% capacity-- say 30 hours a week. So total revenue is 1,500/week. Depending on the nuances of your market space and business model-- 15% of that should go to marketing and sales -- $225/week. That would be a monthly marketing/sales budget of $1,000. We are now in a respectful budget area where you can do something.
Your un-billed time is say 40% capacity or 20 additional hours. The unrealized value of that time is 1,000/week or $4,000/month. If the investment of $1,000 a month increased revenue by $4,000/month would it be worth it to you? Is that an investment you would make?
The business model.
Can your current billable rate cover the marketing/advertising? If not -- what do you need to do to your business model so that it can?
On the Other Hand
Of course, on the other hand -- you could find a relatively bright college student that will do lots of SEO, blogging, Q&A posting, handing out flyers, networking for $300/per month. Maybe they will be willing to do it for $200 and spend $100 of it on PPC advertising?