How about an old film camera? You get "full frame", so a 18mm lens looks as wide as 12mm on digital! E.g.
SMC Pentax-FA J 18-35mm F4-5.6 Lens Reviews - Pentax Lens Review Database
Used Pentax film 35mm SLRs and film-compatible lenses are readily available in the marketplace on this forum.
Also, if you're shooting indoors, lighting may be dim and you might want to invest in a cheap tripod and a flash unit. I got a AF280T - a nice unit that can bounce and swivel.
As for me, I just acquired a Sigma 10-20mm 1:4.5-5.6. It's a digital only lens. It's VERY wide! I took a couple of shots on
Pentax Day with it just outside my home.