From any other Japanese maple. Regardless of the cultivar of Japanese maple, seedlings from that tree will always be considered just generic species Acer palmatum. Some specialized Japanese maple nurseries may have seedling species plants on hand - they use them for grafting rootstock - and now and again, some larger retail nurseries may have more mature straight species JM's for sale.

Otherwise, plant sales, specially like Master Gardener sales, and farmer's markets will often have seed grown Japanese maples available. Be aware of those that they try to pass off as a specific cultivar - the only way you can propagate a named form of Japanese maple is by grafting or, very rarely, from cuttings.

I think the nursery guy meant to say all WEEPING forms of Japanese maples are grafted.
Seedlings from the weeping variety are rarely sold in nurseries.
The seeds don't seem to be near as viable as most of the upright forms, and wouldn't be true to the parent anyway.
Mike