Boat is 99 Baja 272 w/502 MAG. At low RPMs, will hear a squealing noise from the engine. Thought it was the belt, replaced, squealing continued. I have since been told this is probably a circulating water pump issue.

(1) How hard is this to replace for a very basic mechanic such as myself? I found a link on here that shows dis-assembly of that area but putting it to practice is not always that simple.
(2) And where is the best place to get a replacement (not going to Autozone)?

First, determine the cause of the problem/squealing.
It is a simple and straight forward job to replace the circ pump.

It's definitely one of the pulleys and not the belt. Is there a certain procedure to isolate this squealing? Such as, remove the belt and spin each pulley to listen for squealing and then apply some grease to see if it goes away.

It could be a bearing in any of the accessories (sea water pump. circulating pump, alternator, power steering pump). You are not going to be able to grease any of those bearings. Does the squeal change when you move the steering wheel? Given the age of the boat and the fact it has probably never been touched, I'd first change out the power steering fluid and replace with a synthetic fluid/conditioner. I've seen those pumps squeal like pigs and then be quiet as can be after a fluid change and some conditioner.

Squeal does not change based off of steering wheel position but I'll change out the PS fluid anyway considering I'm not sure the last time it was changed. Engine has 514 hours.

Easy way to locate an engine noise is to take a long screwdriver and cup you hand around the handle and put it to your ear and then touch the areas you are investigating. Works just like a doctors stethoscope, been doing that for more years than I care to let on.