The Sienna seats so popular with Econoline owners, brackets sold for, are take out rear and middle row seats. You want a passenger front seat, which is very different. If you go through the trouble and shipping, you'll want to track down a passenger seat. The good news is this; often a wrecker will have a set of front seats, where the driver's side is worn out, but the passenger side still has a lot of life left in it.
For the record, I owned an '06 Sienna XLE (highest trim package), and can't recall a more comfortable seat that I've ever sat in. Not too soft, nor too firm, heated, leather, adjustable, they were awesome. Long stretches doing 10hrs or more behind the wheel, felt like 3hrs.
1995 E350 7.3 Diesel, 4x4 high roof camper, UJOR 4" lift