Contact dermatitis - what can I do besides avoiding the allergen? Any barrier creams as prevention?

Not really. The best thing for patients with contact allergy is to avoid the allergen. Or a physical barrier. The allergen can seep through any "barrier cream" and eventually still cause a problem.
Yes. Ivy shield and ivy block are available. Stockhausen made a series of barrier creams that you may need to buy online. Prescription barrier cream is called tetrix.