Nice to see I'm not the only victim of this...
I bought my car as a rolling chassis so it came with a seat that's like 17" in the hips,and yeah that damn thing is waaayyy too tight for my big ahhh,well you know.
Anyhow here is my list for the "big" poly seats,I looked into the kirkey stuff,nice,and they do offer some wider sizes in a couple of the lines of seats,but they're way too pricey for my budget,and my deal is strictly 10.0 and slower for my current car,the next car will get the nice stuff,and be much faster.
RCI #8000 S ~ 20.5" wide
Summit #G1100 ~ 21" wide
Summit #G2100 ~ 20" wide
Jegs #70200 ~ 20" wide
All are appx. 32-33 inches tall.
All are priced at/around $80.00 when you add in the seat covers.
HTH.