I would look at Korves for seat parts. I bought a few items from them this past summer at the Oliver show and I was extremely satisfied at how they treated me and with the product. They seem to specialize in the two digit Olivers if you look at their brochure or website. 

I've also bought items from AgParts First via mail order, but I've only purchased items for my 1800 and my 520 baler. When the box arrived empty due to mishandling from the postal service, they made it right when it wasn't really any fault of their own and sent me a replacement. I will certainly give them my business again. 

I've read alot of mentions for parts from Obrien County Implement, although I haven't had any dealings with them. They do advertise in Hart Parr Oliver magazine.