On a budget.  

That's a very difficult question to answer
It's like asking which book to buy.
I would look around places like auctions, eBay, Craigslist, flea markets etc. until you find something you fall in love with. Looking for specific items by specific designers can often mean that you miss out on the real treasures
Happy hunting

Perhaps
a pre-loved Eames LCW to compliment your sweet Eames aluminum group chair? Or an Eames time life stool? Or a Saarinen tulip side table?
Just a thought.

Im not
Too into tables I'm more into seating, I'm think something knoll, but all I seem to find are bertoia's.

.
oops,...you mentioned chairs or sofa's.

Yeah.
So any thoughts?

Yeah.
So any thoughts?

A lot of good stuff can be found
at great prices IF you are diligent and patient. There are already tons of tips in the "The last thing you acquired" threads (16 of 'em and counting!) on this board.
But you need to also develop an idea of the style that you like. To just say "designer seating under $400" is very broad.
On the other hand, if you are thinking that you want brand new pieces that retail for under $400---good luck!

I dont want new.
And I said modern? Is that too broad? Kinda new to this.

vintage IKEA
just a thought

You can get
Eames shell chairs all day for 100-150 USD. Look at Eames arm chairs (upholstered) or Bertoia wire chairs. Russell Woodard patio chairs are still well under 300 USD. Plia chairs, Louis Ghost by Panton, etc. Lots of choices for individual chairs under 400 USD. Upholstered lounge chairs are a different story altogether.
Chairs are easy, my friend. Wait until you get to beds.

Im not too fobd of.
Side chairs, but they are growing on me.

still talking about beds
Riki,
You and I have the same dilemma with beds.
What ever became of your search for the Gae Aulenti, Tour table for Fontana Arte? Did you meet with success?

thrift stores and yard sales
most of what I have came from these 2 sources. But do some research and try to put together a punch list of what you are interested in. There's a whole lot of diversity out there, much of which doesn't look all that good in the same room.

"Modern" is pretty broad, yes.
There's Danish teak, there's Eames stuff in bright colors, there's chunkier stuff, there's McCobb, Dunbar, etc., etc. Some of it looks good together but a lot doesn't, as Waffle said. Part of a good design is putting individual pieces together to form a pleasing, interesting whole.
I love a lot of it myself but chose to go with mostly Danish teak. I just admire the rest from afar.