It depends on just how fussy you want to be. i use a pressure cooker to sterilise slants and tubes and small quantities of wort. But eventually you have to put it all into something that is merely sanitised rather than sterilised.. For me the point at which that becomes OK is when its obvious that yeast is massively the dominant possible microorganism.
So... I will pressure cook my yeast slants and the gear I use to innoculate them etc, and i pressure cook the small amounts of wort that i use to initially start the cultures up. To whit, i use the same tubes as my slants for wort - they hold about 15ml and i fill and sterilise a bunch of them.
Each slant (i only use each slant once) gets about 5-10ml of sterile wort added from one tube... Its allowed to kick off and gurgle for 18-24 hrs. Then this is added to a small erlynmeyer that has usually been sterilised too (either in the pressure cooker or the oven, mostly the oven) and i add 2 or so tubes of sterile wort to it. After 24 hours, i am now happy that there will be more than enough yeast to hold their own in containers/wort that have just had the standard boil in place and cool under foil treatment.
The point of this description is that there is nothing in there that requires a "big" pressure cooker. I have a 6L jobbie my mum bought me from Myer or someplace, and its enough to do dozens of slants and tubes at once. Far more than you'll need i6 months of brewing.
Once you get up to an amount of wort thats too big for your pressure cooker... Well, IMHO you are already talking about a yeast cell count thats plenty big enough to manage in just "very nearly sterile" wort. So you just dont need a big pressure cooker for yeast work.