i wish you luck. my mum appears to be losing her mind, and every time she does respond with some sort of reasoning it's different every time. so i'm just waiting. i'll get my first herp eventually. but then i'm working at a pet store so i can get my reptile fix there every few days.
if your mom is likely to be reasonable and hear you out then i definately recomend taking her to a pet store (perhaps scope it out first and talk with the people working there, ask them when a good time would be to come in and what you wanna get across to your mom. i don't know about others, but i know i'd be willing to talk to her and such. i love getting a bit of a crowd around me of fascinate adults and children) and trying to get her a bit of exposure to them. if she isn't willing to even discuss the idea and you don't think she'd have the heart to harm the snake then i'd just sneak one in if you have the opportunity.