Went to a Cardinals game this past season. Early on Tickets were easy to find and plentiful on Ticketmaster. So try and buy in the pre-season.
We bought in Mid-September, and found tickets close to face value on eBay. The reason why there were so many tickets on eBay is the Season Ticket Holders had a 1/100 chance of getting drawn for SuperBowl tickets. So people were buying up bucketloads of season tickets in the hopes of getting SuperBowl seats.
This season there may be a lot more available on Ticketmaster due to the lack of interest in season tickets.
One more tip, make sure you get a good parking pass. We parked in the
Orange Lot.
We spent $30 on the pass as opposed to $8 for a further away lot- so the difference split between 4 of us wasn't much and the experience was just that much better. Oh, and for the beer cooler- this summer when beer companies are selling 24 cans in the big collapsable coolers, pick one up. You can use it for your tailgate beer without having to lug a big cooler from home or buy a $20 one to leave behind there. As we found out too late, they don't sell the cheap foam coolers anywhere.