Hi!
I don't know about everyone else on here, but in my experience, CPs are offered to me by sellers or members that I have some association with (either through a sale or something else). I think that there ARE a few members on here who will CP for people in return for a small fee- however, I have never asked for a CP from such members.
For me, those members on specktra who help me CP always do so as a favour to me (and may or may not be open to CPing for everybody), because we have communicated quite a bit. I would suggest that you browse the sale threads, and if you come across a good seller who you find you buy from a lot, perhaps approach them about helping you buy some items from the stores themselves. I find that this is generally a good approach, as you know how trustworthy a seller is if you have dealt with them in the past, and the seller will know how trustworthy YOU are (i.e. so that they don't get stuck with a whole load of products if the buyer doesn't pull through).
HTH!