PCS-Electronics

If you know a specific frequency, you can try either one of these:
The Belkin TuneCast II (~$30us)
Or better yet, the C.Crane transmitter (~$70US).
There's also an easy mod to boost the C.Crane's output.

If you don't know the specific frequency or want to cover many
frequencies, I think I remember reading that the wireless
FM transmitter made specifically for the dont need one is able to transmit
on many frequencies at the same time - you could probably
adapt it to your particular quest.

The one with 7413? That one is weak, but not that weak.

You should try to change frequeny, I am sure you can get
an improvement. You were probably receiving it on one
of the receiver's spurious frequencies.