if you cant find an MB3756 you have a few options.
you can look for the NTE sub which is an NTE1271.
they are getting about as expensive as the MB3756.
you can eliminate the chip altogether by replacing it with a 7808 regulator and a relay, or a couple of transistors.
here is a link to one way to do it:
(it is in portuguese so you'll have to use google translate if you want to read the text, not really needed though) PY2BBS - Hamradio Page
here is another method:
1) remove the MB3756 chip and save the plastic screw and nut.
2) get a T0-220 mica , 7808 regulator , 10uF 10-50v lectro , 0.01uF
3) twist on the 10uF to the ground and output
legs and the 0.01uF to the ground and input leads then solder on three
lengths of wire to each leg and grease / bolt it to the existing hole
on the wall just above the empty 3756 slot.
4) wire the ground lead to the ground foil , input lead to pin 2
location on board , output lead to pin 1 location on board.
5) 12V coil SPDT relay double sided taped to a wall , put spike diode
across coil (1n4004) , wire common switch contact to pin 1 location on
board , NC switch contact to pin 6 location on board and wire NO switch
contact to pin 8 location on board.
6) wire cathode side of coil to +12V on the radio power switch .
7) disconnect the receive wire from the mike jack and tape up.
8) wire mike jack receive (at jack) to the anode side of the relay coil.
just so you know, i have not tried either of these mods personally, but the first one looks logical, and the second one comes from a source that i have trusted for years, and has never let me down.