Yes, as you've seen, your choices in the U.S. aren't great.

=-- T-Mobile Pay By The Day --=
Per Day Cost: $1.00 (if phone used)
Per Min Cost: $0.10 to U.S., $1.60 to U.K.
Incoming SMS Cost: $0.05
Outgoing SMS Cost: $0.10
MMS Cost: $0.25
Data Cost: n/a

=-- T-Mobile Pay As You Go --=
Per Day Cost: n/a
Per Min Cost: $0.33 / $0.19 / $0.13 / $0.10 * to U.S., $1.60 to U.K.
Incoming SMS Cost: $0.05
Outgoing SMS Cost: $0.10
Incoming MMS Cost: $0.25
Outgoing MMS Cost: $0.25
Data Cost: n/a
* depending upon refill card size

=-- T-Mobile Sidekick Prepaid --=
Per Day Cost: $1.00
Per Min Cost: $0.15
Incoming SMS Cost: $0
Outgoing SMS Cost: $0
Incoming MMS Cost: $0 (I think)
Outgoing MMS Cost: $0 (I think)
Data Cost: $0 (port 80 blocked)


=-- AT&T GoPhone $3 Unlimited Calling Plan --=
Per Day Cost: $3.00 (if phone used)
Per Min Cost: $0
Incoming SMS Cost: $0.20 -or- buy package
Outgoing SMS Cost: $0.20 U.S., $0.25 international -or- buy package
Incoming MMS Cost: $0.25
Outgoing SMS Cost: $0.25 U.S., $0.50 international
Data Cost: $0.01/KB -or- $4.99/1MB package -or- $19.99/100MB package
Note: Optional SMS packages are available: $4.99/200, $9.99/1000, $19.99/unlimited. They need to be repurchased every 30 days.

=-- AT&T GoPhone $1 Mobile to Mobile Plan --=
Per Day Cost: $1.00 (if phone used)
Per Min Cost: $0.10
Incoming SMS Cost: $0.20 -or- buy package
Outgoing SMS Cost: $0.20 U.S., $0.25 international -or- buy package
Incoming MMS Cost: $0.25
Outgoing SMS Cost: $0.25 U.S., $0.50 international
Data Cost: $0.01/KB -or- $4.99/1MB package -or- $19.99/100MB package
Note 1: Optional SMS packages are available: $4.99/200, $9.99/1000, $19.99/unlimited. They need to be repurchased every 30 days.
Note 2: An optional Unlimited Night & Weekend Minutes Package may be added for $19.99 per 30 days. Nights are 21:00 to 06:00. Weekends are 21:00 Fri to 06:00 Mon.

=-- AT&T GoPhone $.25 Simple Plan --=
Per Day Cost: n/a
Per Min Cost: $0.25
Incoming SMS Cost: $0.20 -or- buy package
Outgoing SMS Cost: $0.20 U.S., $0.25 international -or- buy package
Incoming MMS Cost: $0.25
Outgoing SMS Cost: $0.25 U.S., $0.50 international
Data Cost: $0.01/KB -or- $4.99/1MB package -or- $19.99/100MB package
Note: Optional SMS packages are available: $4.99/200, $9.99/1000, $19.99/unlimited. They need to be repurchased every 30 days.

=-- AT&T GoPhone $60 Unlimited Talk and Text Feature Package on the Simple Rate Plan --=
Per Day Cost: "n/a" (effectively $2.00 though)
Per Min Cost: $0
Incoming SMS Cost: $0.20 -or- buy package
Outgoing SMS Cost: $0.20 U.S., $0.25 international -or- buy package
Incoming MMS Cost: $0.25
Outgoing SMS Cost: $0.25 U.S., $0.50 international
Data Cost: $0.01/KB -or- $4.99/1MB package -or- $19.99/100MB package
Note 1: Optional SMS packages are available: $4.99/200, $9.99/1000, $19.99/unlimited. They need to be repurchased every 30 days.
Note 2: The "$60 Unlimited Plan" is $60 per 30 days. If not repurchased at the end of 30 days, it falls back to the $.25 Simple Plan.

General notes:

• eBay is a good source for both cheap, unactivated SIM cards and discount refill cards. You may be able to purchase them before your trip.
• I've listed the extra charges for calling/messaging the U.K. where those charges are easy to find. There may be additional charges beyond those listed.
• Moving from one T-Mobile plan to another or from one AT&T plan to another may be done pretty much at any time via the web or by dialing special codes. Extra packages on AT&T may be added the same way. Some N900 users on T-Mobile bounce to the Sidekick plan just when they need data and stay on another plan the rest of the time.

There are small, regional resellers in some areas and a few large, nationwide ones.

The nationwide ones won't meet your needs. If their phones even have SIMs, those SIMs are tightly tied to the reseller's custom-firmware phones.

You may be able to find a smaller, regional reseller in New York, but most regional resellers tend to 1) resell Verizon, which won't help you and 2) not support any data usage. Also, most of them don't support roaming outside their own limited area, so if you visit someplace outside New York, your phone will be useless except to for emergency calls to the police, etc.