Woodland Scenics makes hydrocal cast portals in both single and double track, and Chooch Enetrprises makes cast resin timber portal. Both are pretty reasonable in price.
I don't know if they are still in business, but a company called AIM made some nice tunnel portals also in N scale.
11:20PM:Just checked Walther's website. They still sell the AIM. So I guess they're still in business.