I just refurbished and gifted a similar Terry to my sister this past Christmas. As the others have said, the front wheel is almost certainly an ISO 520mm "24x1". There are a couple options for tires, though not many. In my searches, I found the Panaracer Pasela, Schwalbe Durano, and a wider Terry-branded tire. The tubes aren't terribly common, but are available, as well.