Where a 'pro' gets it, can't say.
However, I have never really had trouble getting what I need doing a good search through say Google.
Now if you can find the cheapest is a bit of a hit or miss. Have to really do a good search. However, have you considered just contacting the manufacturer to make sure it is the right part? Even with it being out of warranty, you should be able to get them to tell you what the exact part you are needing is. Can't hurt to ask them. Better than doing more damage.
If anyone else has a suggestion on locating the best parts, please chime in.