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Almost any major electronics supplier should be able to get the needed parts.
You'll also need access to an (E)PROM burner capable of burning 1-Mbit EPROMs.
Shop around for your EPROMs, as prices can vary drastically.  ZyXEL does sell
EPROMs for use in its modems.

You need **TWO** EPROMs for a complete set of ROMs. 

***BUY TWO EPROM CHIPS***

NOTE: Radio Shack doesn't count as a major electronics dealer.

Part number to get: 
27C010         1-Mbit EPROM
27C512       512-Kbit EPROM (really old non-plus's)


Speed (of EPROMs) to get:
                |------------------------------------------|
                |   non-plus models     |    plus models   |
|----------------------------------------------------------|
|  512 kbit     |  150 ns or faster     |  100 ns required |
|----------------------------------------------------------|
|    1 Mbit     |  150 ns or faster,    |  100 ns required |
|               |  120 ns recommended   |                  |
|----------------------------------------------------------|

John Weidman (dk139@cleveland.Freenet.Edu) has bought the 100ns EPROMs (part
number TMS27C010A-10JL) from Newark Electronics (1-800-367-3573 in the US and
Canada).

Ackers Mattias (d2matac@dtek.chalmers.se) has used "ST M27C1001" Signetics 
Thomson (??) 100nS EPROMs and they work fine with his E+ modem.

Andrew Jackson (jackson@stc.nato.int) has also used SGS-Thomson 27C1001 EPROMs.
They work. In his experience, (but not confirmed) these devices apparently do 
not require the external links from Vcc to pin 1.  They cost about NLG 18 each 
(about $9US) around September 1993.

David Bowerman (1:153/290) [Fidonet] has used the Macronix 27C1000 and 
AMD AM27C010 parts as well with no problems being reported.  You do have
to be careful though since the part numbers for that size of EPROMs are a
regular dog's breakfast.  One company's 27C010 is another's 27C1000 and
yet another's 27C1001 -- double check the cross references and make sure
that the chip doesn't need the Vpp pin to be tied to Vcc (+5V) during
normal operation.  The ZyXEL modems don't even have a pin in the socket
at that pin (1) -- a workaround is to solder a 22K resistor from pin 1
to pin 32.  This allows the pin to be pulled high but doesn't allow
enough current to flow during programming to cause problems -- at least
the programmer I'm using doesn't seem to mind.


Note for the inexperienced:
        Using faster ROMs will not make your modem go faster.  The speeds
        recommended are those found to provide good reliability and 
        operation and getting faster ones will not provide any further
        speed enhancement.

Note for the lazy:
        Robert McKeever (mckeeveb@sfu.ca) has purchased a large quantity
        of 27C010's and would be willing to sell some at a reasonable price
        (plus shipping) for those who don't want to shop around.  E-mail him
        for details.

        Robert McKeever (mckeeveb@sfu.ca) would even be willing to have them
        burned for a few bucks more.

        David Bowerman (1:153/290) [Fidonet] at Frog Hollow BBS in Port Moody,
        BC, CANADA, (604)469-0264 BBS (preferred), (604)469-9952 voice will 
        burn a set of EPROMS.  He is asking $5 if you supply the EPROMs, 
        or $25 if he supplies them.  His address is 112 Brookside Drive, 
        Port Moody, BC, V3H 3H4.  Please make sure that you state which 
        modem model you have.

        Sean Foley (1:202/1609) [Fidonet] (18:701/1609) ZYXELnet at System 
        Support BBS in San Diego, CA, USA, (619)563-9207 BBS, (619)563-9311
        voice will burn a set of EPROMS.  His address is 2869 Spruce St., San
        Diego, CA 92104, USA.  He is asking for $5 (shipping included) with 
        the exchange of the old set, or $25 (shipping included) for a new set.
        Please make sure that you state which modem model you have.  The EPROMs
        are burned on new TI 27C010's that are matched to the speed of your
        ZyXEL model.  Each EPROM is verified twice (buffer and CRC) before it 
        is shipped.  He will also include the release notes and such also.

        Wes Newell (1:124/7028) [Fidonet] at Wylie Connection in Wylie, TX,
        USA (214)442-0388 BBS will burn a set of EPROMS.  He is asking for
        $5 (shipping included) for reprogramming the old set, or $15 
        (shipping included) for a new set.  Please make sure that you state
        which modem model you have.  Send him the modem model, the money,
        etc. at 1213 Devonshire, Wylie, TX, 75098 and he will send you a 
        set of EPROMs with the latest ROM version.