sex, erection supplements

ive had erection problems when i go to have sex with my fiancee. its fine while in foreplay, but once i get to sex its not hard, semi soft. i dont smoke, dont drink, i dont have alot of stress in my life.i dont masterbate often.just was wondering what natural supplements you recomend for sex and erection. ive read tons of stuff on this, but nothing as a definitive list as to what to take.just was curious as to what to take. im on centrum right now.
can i go to walmart and buy these or do i go to GNC or health store. any help would be greatly appreciated

I am a 30 year pld boy and have problem of slow erection. Some times its ok but at times I can't achieve an erection at all. Please advice me on what I could do and are there any suppliments available to boost my performance. The natural ones would be better.