Greatings. I am new! (thanks for the applause)
I read the pdf guide! (thanks for the applause)
And I have been through the forums, but I have some simple questions.

I have a sheet magnifier, that I have measured to be 220mm focal lenght.
How do I find a triplet lens of similar length? Ebay listings don't really give this.
The bigger the lens the better correct? 80mm for the lumenlab triplet.
But focal length is not determined by this correct? If I go to a yard sale
flea market, etc. what do I look for when I find a lens appart from holding
it up and measuring focal length with a light and tape measure?