GSP advice needed

[Cinder]

I am looking forward to having a single GSP in a 29 gallon aquarium. (If that size is too small, should I get a Figure 8 or a Green Topaz instead?)
My first question is: Does anyone have a reputable online store where I could purchase a GSP? The only place around me that sells them is Walmart. I know that Walmart has a poor reputation regarding the care they give their fish, but the two ladies who work in the fish department are actually very caring and knowledgeable. They feed pest snails to the puffers and try to make sure that each fish gets his share. They also asked me to bring home a VERY pregnant platy so she could "have her babies in peace". I believe that they do the best they can for the fish, given the tools they have.
Walmart puffers are in fresh water. Should I keep my cycling tank as fresh water and slowly add Instant Ocean when I get the fish?

How do you increase the salinity of aquarium water with a fish in residence?

There are no worries about my ability to feed snails on a regular basis. I have a huge supply of ramshorn snails in my 55 gallon FW aquarium.

I look forward to any and all advice!

[Pufferpunk]

I think if they seem to care about their fish & they look healthy, then you could probably buy one from Walmart if there are no LFS near you, that you could support.

library/puffers-in-focus/an-introductio ... d-puffers/
viewtopic.php?f=8&t=19348

[Cinder]

Thank you so much for the information! It is all so very helpful.