Bikefabsupply sells BMX specific tubing. That would be a good starting point:
- Downtube: 34.9MM Ø 4130 TOP/DOWN TUBE 1.2 X 650MM — BICYCLE FABRICATION SUPPLY
- Seat tube: TRU TEMPER BMX SEAT TUBE 27.8MM Ø X 1.2 X 223MM FOR 25.4 SEAT POST ( WHILE SUPPLY LAST) — BICYCLE FABRICATION SUPPLY
- Headtube: HEAD TUBES MACHINED BMX STYLE IS42/42 — BICYCLE FABRICATION SUPPLY
From what I see, most people laser cut their dropouts.
After doing a bit of research, I would reach out to BMX builders directly with questions. People are all friendly and willing to help.
Someone on the forum recently posted about S&M’s Factory Friday videos. I have found them both entertaining and helpful, even though I don’t build BMX bikes!
Laird does some crazy volume: https://www.lairdframe.com/
Kahl is a smaller BMX builder: Jackson (@kahlbikeco) • Instagram photos and videos
Happy hacking!