The Wen Electric Company was purchased by Weller in the early 1980s (as I remember).
Weller made tips for the Wen 75 for another 10 to 15 years after the merger -- but then discontinued their production.
They do occasionally appear on eBay and various surplus outlets.
You can contact Weller USA in Apex, NC ... to see if they may have a few still in stock with the Repair Center.
PS I owned a Wen 75 in 1970s ... it was my first soldering iron (gun).
Wen Electric was based in Chicago, IL.
The WEN 75 soldering guns were sold by Coast-To-Coast and True Value hardware stores in the 1960s and 1970s.
I would imagine Weller has no parts -- these tips as NOS are getting RARE.
I would HIGHLY SUGGEST you place a WANTED at QRZ, QTH, and Craigslist .. as well as a KEEN EYE during Hamfests.
Local hamfest 5 years ago ... I picked up 7 soldering tips for the GE Soldering iron (6 volt) used by the Heathkit GH-17 solder station (long discontinued) for $1.00 GE stopped manufacturing of those tips in the early 1980s ... sold all of them to a group of Texas Heathkit collectors/users .. for last Heathkit catalog price (more than $1).
These Wen 75 soldering tips DO APPEAR on eBay ... as long-time hardware and electrical distributors clear out old inventory.
It can not hurt to CALL .......... ACK Electronics in Atlanta, GA or Birmingham, AL .. they were clearing out some inventory last year on eBay and
I picked up some nice legacy Cushcraft items (not currently mfg. by MFJ). http://www.acksupply.com/