can I buy pH test strips for checking star san and other things somewhere other than my LHBS. Like something labeled for a pool or fish tank?
As long as they’re just for checking StarSan, anything in the correct pH range should be fine.
pool pH strips usually don’t go below about 6.0pH. You can get them online but honestly I’ve never had any luck with those damn things. The 0-14 strips are about $20 per hundred. I gave up and got a pH meter. $75 well spent IMO.
is there any reason to check the pH of a finished beer either before priming or after its carbed?
Well excepting to know what the pH is at those points, I’m not sure there is a good reason to measure beer pH. The yeast have a lot to do with the final beer pH.