Does anyone know where to obtain T8 Sockets like these?

There exist standard "troffer sockets", those are T8 size and mount with 2 plastic posts that stick out of the back of the socket. But they are side by side, not top and bottom like here, and looks like they won't match the holes in the bracket anyway - to use them you will have to drill new holes in the bracket

Something like :

AAG Stucchi :
http://www.aagstucchi.it/products/category/147/ - click on A PASTIGLIA
The most slim ones (what i think you need there) are : http://www.aagstucchi.it/wp-content/uploads/data_sheet_pdf/it/910.pdf

Vossloh Schwabe :
https://www.vossloh-schwabe.com/fileadmin/user_upload/Download/Produktkatalog/VS-Component-Systems_2014_EN.pdf - pages 217..220
The best type are 507133/507134 (p 219), second after them are 509162/509164 (p 217)

BJB :
http://www.bjb.com/index.php?pid=263649&lid=10

Vossloh Schwabe make some similar sockets that can be mounted with screws (se on the same pages), but the screw positions still are same as for the plastic ones. If you want to avoid drilling the bracket you can try mounting those diagonally with 1 screw... (not the best option but it will do) - best i think is 101740 (p 218)

And they make some sockets of similar design to your originals (end of p 220) but they are T12 size

Note the socket thickness, some of the sockets are made in few versions like 10mm and 13mm thickness and you have to choose the ones that match the distance between teh brackets

Dont know where you can get them, here i can intermittently find them at wholesalers (there are 2 local ones that i check with, most of the time one of them has what i want)