The requirements makes it a bit difficult to find a substitute. If you just needed a solid color, sure, but two solids and an ombre... hmmmm
Only thing I can think of is Lorna's Laces Shepherd Bulky... pick two colors (like Grapevine and Lilac), and a blend that works with them (like Purple Club).
Or, knit with two strands of worsted (to make bulky weight). That way you could knit with two strands of purple, two strands of lavender, or a strand of each to get your ombre color. Just one suggested yarn is Lambs Pride because of the color selection. Note: If you go this route, be extra sure to check and double check your gauge!