I don't have a company to suggest directly, but would just submit this for your consideration when Choosing such a company.
Before signing up for the services, Ask them if they are Aware, and Dealing With the problem in the case of both y and g ads, as it applies to the many paid to read email programs now accessing those ads via the domain parking companies.
If your service provider is Not on the ball with that one, they are going to Lose a Ton of money to Cash Paid Clicks on your ads.
Of course this is Very dependent on a number of other factors, some niches are hit far harder than others, simple keyword phrases are the most common ones on the parkers portal pages, and other factors apply as well, but you need to be careful I think to try and find a company to represent you that is at Least Aware of the many thousands of monthly clicks the ptr sites are sending, Especially the new 'forced search' sites, as they are now called, they only send the paid links to the members of their programs that Finish the bogus searches for them, so the ctr rate is Very High on them.
I know that some here in this, and other advertising and related forums have suggested in the past, that the cash paid clicks from the ptr sites is not a big issue, but if You are One of the Advertisers, that is stuck with Hitting a Keyword that is front and center on one of the parkers, and there is a domain parked there that is in Your Niche, and Sent as a Paid Ad 7 Days a Week, to a Couple Thousand Hungry Clickers, it's gonna Rip through your budget like the cash machine it was Designed to Be.
Used to be this past few years, that the ptr sites Only hit g and y Occationally, and relied on the 2nd tier ppc companies as their primary income stream. This is No Longer the Case. They more and more are Abandoning the 2nd tier, having watched the bids fall so low they can barely make a profit from them. Now, they have found a New Goldmine, in the parking companies, as they have 3rd party indirect Access to the High Paying g and y ads they so long have wished for.
So my Caution to you, is to have a company Represent your campaigns that is On To This One.
There Are a small Handful in the ptr and related forums that Are doing a good job of Reporting much of this to the parking companies, (an Unfunded and rather Unrewarding Undertaking btw), but they hold Little Weight, not being ppc/cpc advertisers Themselves, so it's Important I think, that Your Company you are dealing with to manage campaigns is on top of this issue on your behalf.
Your ppc Company Itself, May have at least Some targetting options, allowing you to filter out, or 'site-target' some of the Better of the parking companies, your Management Company, should Also be on the ball in This regards, Knowing which to permit, which to filter out, as the ptr peeps are finding the lowest in the barrel they Can, as they Lose accs with the Better ones, so the cat and mouse game is an Ongoing battle, if your representatives are not on to the game, it Will Cost you in the end, imho.
Good Luck to you, and be careful out there. sorry if this was offtopic.