I live in the US, but this is what I do.
1) Consignment Shop - a store that sells secondhand items.

2) Go onto 'Craigslist' and check out the 'free stuff' category or 'household items', 'office supplies',etc.

Hi, if you click the garden centre box, just below where it says ''your options'' it will take you to the garden centres who advertize on GoY, just click on where it says ''go to garden centre'' and then click on ''accessories'' itwill give you lots of options, just select greenhouse staging, and it will show you the different options available, Derek.

Hi Vicki, we have just bought metal ones (a shelf and staging) online from the greenhouse people, Alton. Plenty of fixing instructions given - good luck.

also don't forget places like amazon to get price comparisons.

I have a Robinsons' green aluminium greenhouse and they sell benches and shelves.
https://greenhouses-uk.co.uk/elite-greenhouse-accessories/shelves.html

Whoever you bought your greenhouse from should have such accessories that will be the right size for yours.

www.twowests.co.uk - not cheap, but comprehensive and long-lasting

Look at your local 'Buy, swap and sell' pages on Facebook and ask if anyone has any.

I used plastic garden tables for a long time, found them easy to wipe down too, then someone we know gave me some staging but I think the garden table were better :o))

All my shelving comes from them old mini greenhouses, the one's that people buy for £20 or so, with the plastic covering.The plastic never lasts for a season or so. I have been given these by friends and neighbors over the years. Try boot sales.

Thank you to everyone for their suggestions. xx