I recently bought a round 11" cake rack from a kitchen collection store for about $2.00. I read that suggestion somewhere on this site. I also read to take it out of the water when finished because it can rust.
Teri

Here are some stainless steel wire racks that should last a lifetime. The smaller one is about 8 inches in diameter, and tehlaregr is a bit over 12 inches. My orginal rack started to rust badly, so I replaced it with one of these. My canner is a bit under 12 inches, so I bought the larger one, removed the two handles, and bent the outer wires in so it would fit my 11 inch canner. Mine is a steam canner. The cake racks all seem to rust as they have very thin coatings of chrome and it wears off quickly in a water bath.

Here is a link that might be useful: Canning racks stainless steel

For your pressure canner - the manufacturers website will offer the bottom racks for all the different sizes of PC's they make so just order the one for the size you need.

For example if you have a Presto 23 quart and its rack is 2" too big for the bot you want to use just order the plate for the Prest 8 or 10 quart.

But that is the expensive way to go probably because as temiha said there are many alternatives - cheap - that will work just as well - like an old 10" metal pie pan with some holes punched in it and set in upside down in the bottom of the pot. ;)

Dave