Green Nomad..yes Hua Hin Bike DO sell clothing, & yes padded-shorts (or 'nappy-pants' as the wife calls 'em) are available in HH..she got me some for Xmas, think she paid about 600B for them..but they do work well for me..I average 30-50kms every other day on/off road when in HH.
As for cycle shirts at Hua-Hin Bike..yes they have heaps of them, but you need a large-ish size..& as they mainly cater for the Thais your choice can be limited.
Another factor to consider is wearing a long-sleeved cycle-shirt..I do this on most morning trips 'cos it's damn hot by 10/11.00..but, on my last visit to HH Bike their L/S shirts were quite thick & to me not suitable.(1 option is short sleeved shirt+ those tattoo arm-stockings to keep the sun off..

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As yet I've never needed to venture into Bangkok & check out the large cycle shops there..?
This Autumn while I've been back here in the UK I've shopped online at 'Chain Reaction'..good site, great price range, they sell the baggy shirts/jerseys as well, (I just got a Kona loose/thin jersey from them)..always stuff on offer..IF you are truly stuck for shirts & pants & see some online, then send to me & I'll bring them out with me to HH mid Nov.

U.S.