Like buying anything, outsourcing the construction and/or painting of your precious little men is down to you: how much you willing to pay ? how long are you willing to wait ? There will be a going rate, but this could open to negotiation.
Using any commercial painting service is a relationship; you will only get out of it what you put into it. (yawn inducing but true)
Basically, there are two ways to go: a full on commercial painting service operating as a company or co-operative. Or a single artist. What you get with the commercial painting service depends on your communication with them; I think that there are a few things you as a client need to give a little thought to:
The level of communication you want; Do you want WIP photos every week ? Are you happy with a 'this is what it is.' photo before you pay and they post ?
The tighter the brief you give, the more likely it is that you will get what you want. Don't shy away from what may seem to be an absurdly panikity level of detail; the painter(s) are not inside your head and don't know what you want, so the more information you can give, the more likely you will get what you want.
The last thing is that you have a slightly enhanced probability of getting something done to a deadline - if you need the army done before blogwars, then the company or co-operative, having multiple painters, is going to be more likely to meet your deadline.
If you manage to find a single painter, an artist with whom you can establish a rapport, then you may get to the point where they 'tune in' to your style and peccadilloes and/or you trust their creativity and ability to produce something that just looks great and that you will be happy with and didn't know you wanted before it turned up.
But this takes time. And means that you will have put a few commissions their way in order to get to that point.
Another thing which is obvious but took me a little time to learn is that every person who does this does it because a) a lot of other people have told them that they are good enough. b) they love painting figures, even getting a kick out of presenting you with 140 iggies. That you have made them paint all the same. And c) if they muck it up they will struggle and persevere with your commission because their own pride/stubbornness will make them. All of which are a win for you as client.
So, to get personal; Why do I use these services ? Time is one thing. I can paint iggies. But I cannot find the time. And if I'm paying someone quicker (or who has the time) and better (let's face it), the scheme can be a bit more exotic. So time, yeah, a pro painter can do in a weekend what takes me a year.
Skill level is another; my best effort will produce what I sometimes get back as "table top quality". Now, I realise that I could take steps to improve my painting (like doing some, for a start). However for reasons I outlined some time ago, I don't want to, because I have other things to do with my time.
But to sum up being a paying client:
If you have a deadline, colourscheme, notion, design, preference, cost limit, expectation, desire, requirement or fetish that needs fulfilling, then it is down to you as client to let the painter know.
So, there you have it. My take on getting someone else to paint your little men.