A $12 RTL-SDR USB stick will 100% work for most GSM rx (won't hit the 1900+MHz bands without a downconverter, but that's why you want tx - you'll use it to force the phone onto the 800-900MHz bands anyway, the top half of the 915MHz ISM band in the US if you have a HAM licence and are trying to not break _too_ many laws at once).
Gives a good overview of why you need tx to capture an IMSI - without forcing a handset into transmitting non-encrypted, you only get the TMSI (unless you can crack the crypto - and why bother, when a base station can instruct a handset to just not encrypt anything?)
This DefCon talk: https://www.youtube.com/watch?v=fQSu9cBaojc
Gives a good overview of why you need tx to capture an IMSI - without forcing a handset into transmitting non-encrypted, you only get the TMSI (unless you can crack the crypto - and why bother, when a base station can instruct a handset to just not encrypt anything?)