Does your current ps3 have 3.55 or lower installed? Because then you could just jailbreak that, then buy a new one for online.
If not, depending on the individual store, it is most likely that the playstation wont be on the latest firmware. There are possible ways to downgrade firmware, but from what I can tell it's a bit complex.

you cant go online on 3.55 cfw....and if you want to buy a new ps3 to put cfw on it i think you just have to hope that you will get one on 3.55 or lower

Tbh, just buy one of craigslist that has lower FW, make sure that you test it where ever you meet so they don't rip you off. I've heard from people that its still 3.50 in newer consoles, but I would doubt it. If Sony had any brains it would be 3.60+ now. Depending on the store's return policy, if you can return it after opening it, just see the FW.

our ps3 at walmart (where i work) come stock with 3.56 so im guessing other stores do to you could try game stop try to get a unopened 120 gb or a prowned or maybe check ebay/ craigslist