Quick background for you...
MLS (multiple listing services) are generally used by agents for various regions. Thus you will find an individual MLS for every county more or less. The services are used by that local real estate association to post listings. Additional data such as SOLDs, (to be used for comps) but also expired, cancelled and pending listings are in there. Frequently one only uses SOLDS for comps but further analysis is always helpful For instance a very high number of cancelled and/or expired listings helps analyze the flux in a submarket. Furthermore I like to use ratios such as an ACTIVE/PENDING ratio to let me know how what is happening. Obviously a submarket with 25 active listings and 10 pendings is in better shape then a submarket with 25 active listings and 2 pendings.
As the web grew enterprising 3rd party providers saw a great business niche and the development of IDX grew. IDX is the process in which every MLS database is provided to a third party entity. That entity can use the database and advertise it on the web. Thus you have the birth of your Zillow, Redfin, Realtor.com, etc.... The MLS databases that these entities are provided get scrubbed a bit, (some information is removed) and then these entities can provide the database to consumers in their own framework.
Now some of these entities are national, they get EVERY MLS database and provide that. Realtor.com for instance. Other entities will get the MLS databases for only certain regions, like Redfin I believe has databases for a few counties around the nation.
So back to the question, should you pay a realtor for the comps. Alot of the work a realtor does is legwork and alot of it is actually free. If you hunt around enough you will find someone to do it for you. Generally more established realtors will not because they know you will just use the data and never use them. Hungrier realtors, especially those starting out may indeed get you a feed going, or service individual requests. To me it does somewhat depend on your intent as well. If you have no intention of using them, then yeah I guess maybe offering them some money is morally nice to do. Of course it also depends on the types of properties you are looking for and what is in it for them. I think the public is pretty misinformed how much work a realtor actually does for free. How many times they get backdoored by clients, how small a slice of commission they actually get. So on the one hand I think it is nice for a realtor to get paid for comps but on the other, it is simply part of the job.
I agree with the other suggestions on the referenced thread. Getting a license is a good idea but depending on your state and/or county may be a little more time consuming then you have time for. Once you have a license you need to be under a broker and then you can go to the local realtor association to join an MLS. It is not free. Depending on where you live you will most likely have an annual fee, quarterly fees, and you will need to join NAR, national association or realtors. You will also have to join your local association, and again depending on the state you are in, your state association. For instance in California you have to join your local association, then CAR (California Association of Realtors) and NAR. Then you have your quarterly fees as well.
In the end it may indeed be worth paying someone or finding a 3rd party service.