I just ordered my first Oncidium on Amazon from The Orchid Gallery. It cost almost $25.00 with shipping so it's about as much as a Phalaenopsis from Lowe's. Read nothing but good things from reviews but can't fully vouch for them yet.
Oncidiums are my favorite to grow because I love how the plants look when not in bloom. I am not sure where you live but I have occasionally found some great very large specimens at the Fresh Market. I recently bought a Miltonidium Goliath's Spire there for 17 bucks and it had nearly 9 pseudobulbs.
One ebay, I have bought two oncidiums from Shawnlund1 and so far so good. The plants shipped quickly and arrived to me in pretty good shape. Outside of trimming down the leaves on one bulb, I haven't had any issues. The plants are usually less than 25 bucks including shipping from this seller. I have not tried any other sellers yet but plan to very soon so I will keep you updated.
I've bought plants from The Orchid Gallery as well and I was happy with my purchase.
ShawnLund1 is another I've used, also happy with the purchase.
There's also Jewell Orchids, Kawamoto (haven't bought anything from them but they have a really good reputation), Nia's Orchids, Big Oak Orchids, seller: cowboyflowerman (Secret Garden Rare Plants) all on eBay. All but Kawamoto I have bought from and been happy.
I bought an big Shery baby at trader joes in October and I have a 12" spike. It was $14 and had 6 mature bulbs. I think it was a great deal I love it.
most of my orchids come from Trader Joes but the little Tolumnia I got ebay. I really like being able to see the plants I buy check the roots and all. I have had good luck with Trader Joes. Yes 90% do not have tags but I don't care about that I am not showing or breeding.