Here is a list of shops in your area: http://repairpal.com/directory?address=77662+-+Vidor%2C+TX&car_brand_names=Volvo
This is a very simple repair on this vehicle, so any shop can perform this job. I advise using only Volvo spark plugs, I've seen too many issues with aftermarket spark plugs on these models, and it just isn't worth the few dollars you'll save.
change them yourself. it is very simple and quick to do. I use NGK iriduim plugs on my s80. They cost a bit more but worth the money. remove the torx screws on plastic coil cover on top of eng. remove oil fill cap and then removed plastic cap. reinstall oil fill cap. remove 1ea bolt from each coil pack. disconnect connector from coil pack. Pull coil pack with boot from spark plug. remove spark plug from head. Becareful not to drop anything into the spark plug well once you remove spark plug. Inspect coil pack for any damage to include the boot. Replace boot or coil if need be. Should only take you 10 mins to do from start to finish.