Pollen patties and pollen sub patties (no real pollen) are used to build up bees (keep them raising more brood) when little or no pollen is coming into the hive.
This part of the country I would say (for me) the most bang for the buck would be in fall, August on upto possibly November.
Who sells the best? No comment, I am sure someone else on this forum has the answer for you !!? If you buy pre-made realize that you are paying for sugar and factor this in.
If you can find someone locally buy it from them. I have found shipping is cheapest from Mann Lake CA to here so if you need a 50lb bag of dry sub I think they carry beepro. I have used in the past, the bees take it, I do not know personally which is best. ScientificBeekeeping.com Randy Oliver has a good write up on comparing patties under "bee nutrition" or "fat bees skinny bees" or something like that...
This winter/ spring in WA pollen is coming in (when the bees can fly) but if you inspect hives and find little to no pollen stored in combs then it certainly couldn't hurt.
After swarming... Well maybe but mainly used late fall (ie sept, oct) and/or late winter (jan, feb, MAYBE march? ...but this is an early spring in the PNW) to build up hives but realize once you start feeding you should keep a close eye on stores, good chance you will have to feed syrup all the way until flow begins.
Personally only time I feed sub during the summer is before grafting or queen mating flights in nucs.
hope this helps