Chris,
I can't say I've had problems with any suppliers in general. I usually try to find more than one source for the part I'm looking for before buying, if possible. Of course, some are more expensive than others.
Usually shipping is a big factor in the cost of getting the parts so that is something to figure in when you are shopping. A lot of dealers ship via UPS so you won't be able to have it sent to a PO box. I got a part off of eBay once and noticed the cost of shipping was way less than I was charged. When I contacted the seller about it, expecting a refund of sorts, he said it was for 'labor' since it was a pulled part. Maybe my only bad experience in buying parts - just watch for the shipping.
Don't forget the Classified sections on the MV sites: G741.org, G503.com, SS, all have them. Post what you are looking for and often someone will chime in that they have one collecting dust that they will let go. I've had good luck with this also since I can usually get very good photos of the parts before buying and they can usually ship the least expensive method.
For M37 parts, I have my favorites that I go to first based off experience in the past, just a personal preference of sorts. But all in all, none that I wouldn't deal with again if they have something I'm looking for.