What do you mean by modelling and simulation services??
Artisitc impressions, Design, Static analysis, dynamic analysis. By stock models i take it you mean 747, 737, A380 etc etc. If you do then you need to be aware that there are many variants of each generic aircraft type, and furthermore, each user (British Airways, virgin etc) has there own layouts which change on a route/seasonal/new image basis. The OEM's will give a general layout which is acceptable, and this includes changes that the airlines can make without re-evaluation of the airframe. The OEM's also provide specific tailored requirements (for a cost). Third party design organisations will design changes for you on a modification basis.