3GS on 4.0.1 Won't run apps, How can I fix this?

[Phoenix]

I have a 3GS running 4.0.1, unfortunately I allowed all the apps to update, now some of them no longer run stating "This app rquires iOS 5"

I have been told a 3gs runs like crap with IOS 6 but how can I update to iOS 5 when its no longer available for download?

What are my options and how can I resolve this without updating to iOS 6?

Thanks