I recommend the AG Russell Cowboy or the large Hunter. The Gunstock Lockback is also good, but the blade length is just below what you desire (assuming that the 3 1/2" you mention refers to blade length). These knives are affordable, yet their quality is top notch. Actually, this is the only brand of slipjoints I would buy sight unseen, as I don't trust quality control of other current factory production knives.
If you want cheaper, try Rough Rider, but I don't know if they have models with the blade length you require. Opinels are cheap, too, and have some fans, but I'm not one of them...