Easy, as people have said, local papers, Ebay, Gumtree, supermarket and shop windows (cards) and local auctions.
I furnished a 2 bed room house for under a grand including washing machine, tv, stereo, sofas, dining table and chairs, beds (bought a new mattress)
I got a 4 seater and 3 seater huge soft sofa brand new at a local auction for £300 as nobody wanted the purple one
Washing machine, tv, stereo, iron, kettle was from the same place and came in at another £300.
The other £400 was spent finding local bargains for sale especially big stuff on Ebay which is collection only.
Even now, I check on Ebay for bargains like that. That's how I managed to get a mint full set of Wharfdale diamond speakers including subwoofer for £35.
Furtinure is especially cheap and devalues as soon as you buy it from a shop. Take the Alva computer deak/cupboard. Ikea sell them for £220 but you can find many mint ones sold locally for under £50.
We have recently been looking for a couple of leather sofas on Ebay for my "boys toys" room and there are many to choose from under £100. Some of these were near £1000 when new.