Hyundai Accent GLS Rear Shockers Absorbers Replacement. Need Help?

I have 2003 Accent and now i am at an stage where i have replace my rear shock absorbers as soon as possible. I inquired at Hyundai and price quoted to me was 4K each. Now, I am not sure if it is a right decision to go for Hyundai service (as i find it over priced) or to get it done outside. I have installed CNG so also need suggestions on any alternative shocks which will not affect the ground clearance when the rear sit is fully occupied with full gas tank .

Please suggest me what to do now to fix this problem.