Honda Civic SI performance parts are available from Stillen, Pro CIvic, SFX Performance and Cor Sport USA for online order. SFX Performance and Pro Civic offer phone orders. Mail in orders are available from Pro Civic.
Stillen offers in house experts for questions about parts selection or installation. Pro Civic saves a vehicle model and only displays parts that are compatible with it. It also lists the installation difficulty of each part in the description. It offers an upgrade guide showing how to get more power. The guide goes over different types of upgrades such as exhaust and engine tuning explaining how each upgrade increases performance.